Improvements to message-syntax-checks docs and options

* doc/misc/message.texi: Move the manual entry from the "News Headers"
section to the "Message Headers" section, as it is generally
applicable, not just to News.
* lisp/gnus/message.el (message-syntax-checks): Add an explicit alist
type specifying the valid options; point to the manual for more
information.

doc/misc/message.texi

@@ -1792,8 +1792,89 @@ member list with elements @code{CC} and @code{To}, then
 @code{message-carefully-insert-headers} will not insert a @code{To}
 header when the message is already @code{CC}ed to the recipient.
 
+@item message-syntax-checks
+@vindex message-syntax-checks
+Controls what syntax checks should not be performed on outgoing posts.
+To disable checking of long signatures, for instance, add
+
+@lisp
+(signature . disabled)
+@end lisp
+
+to this list.
+
+Valid checks are:
+
+@table @code
+@item approved
+@cindex approved
+Check whether the article has an @code{Approved} header, which is
+something only moderators should include.
+@item continuation-headers
+Check whether there are continuation header lines that don't begin with
+whitespace.
+@item control-chars
+Check for invalid characters.
+@item empty
+Check whether the article is empty.
+@item existing-newsgroups
+Check whether the newsgroups mentioned in the @code{Newsgroups} and
+@code{Followup-To} headers exist.
+@item from
+Check whether the @code{From} header seems nice.
+@item illegible-text
+Check whether there is any non-printable character in the body.
+@item invisible-text
+Check whether there is any invisible text in the buffer.
+@item long-header-lines
+Check for too long header lines.
+@item long-lines
+@cindex long lines
+Check for too long lines in the body.
+@item message-id
+Check whether the @code{Message-ID} looks syntactically ok.
+@item multiple-headers
+Check for the existence of multiple equal headers.
+@item new-text
+Check whether there is any new text in the messages.
+@item newsgroups
+Check whether the @code{Newsgroups} header exists and is not empty.
+@item quoting-style
+Check whether text follows last quoted portion.
+@item repeated-newsgroups
+Check whether the @code{Newsgroups} and @code{Followup-To} headers
+contains repeated group names.
+@item reply-to
+Check whether the @code{Reply-To} header looks ok.
+@item sender
+@cindex Sender
+Insert a new @code{Sender} header if the @code{From} header looks odd.
+@item sendsys
+@cindex sendsys
+Check for the existence of version and sendsys commands.
+@item shoot
+Check whether the domain part of the @code{Message-ID} header looks ok.
+@item shorten-followup-to
+Check whether to add a @code{Followup-To} header to shorten the number
+of groups to post to.
+@item signature
+Check the length of the signature.
+@item size
+Check for excessive size.
+@item subject
+Check whether the @code{Subject} header exists and is not empty.
+@item subject-cmsg
+Check the subject for commands.
+@item valid-newsgroups
+Check whether the @code{Newsgroups} and @code{Followup-To} headers
+are valid syntactically.
 @end table
 
+All these conditions are checked by default, except for @code{sender}
+for which the check is disabled by default if
+@code{message-insert-canlock} is non-@code{nil} (@pxref{Canceling News}).
+
+@end table
 
 @node Mail Headers
 @section Mail Headers
@@ -2072,88 +2153,6 @@ Other variables for customizing outgoing news articles:
 
 @table @code
 
-@item message-syntax-checks
-@vindex message-syntax-checks
-Controls what syntax checks should not be performed on outgoing posts.
-To disable checking of long signatures, for instance, add
-
-@lisp
-(signature . disabled)
-@end lisp
-
-to this list.
-
-Valid checks are:
-
-@table @code
-@item approved
-@cindex approved
-Check whether the article has an @code{Approved} header, which is
-something only moderators should include.
-@item continuation-headers
-Check whether there are continuation header lines that don't begin with
-whitespace.
-@item control-chars
-Check for invalid characters.
-@item empty
-Check whether the article is empty.
-@item existing-newsgroups
-Check whether the newsgroups mentioned in the @code{Newsgroups} and
-@code{Followup-To} headers exist.
-@item from
-Check whether the @code{From} header seems nice.
-@item illegible-text
-Check whether there is any non-printable character in the body.
-@item invisible-text
-Check whether there is any invisible text in the buffer.
-@item long-header-lines
-Check for too long header lines.
-@item long-lines
-@cindex long lines
-Check for too long lines in the body.
-@item message-id
-Check whether the @code{Message-ID} looks syntactically ok.
-@item multiple-headers
-Check for the existence of multiple equal headers.
-@item new-text
-Check whether there is any new text in the messages.
-@item newsgroups
-Check whether the @code{Newsgroups} header exists and is not empty.
-@item quoting-style
-Check whether text follows last quoted portion.
-@item repeated-newsgroups
-Check whether the @code{Newsgroups} and @code{Followup-To} headers
-contains repeated group names.
-@item reply-to
-Check whether the @code{Reply-To} header looks ok.
-@item sender
-@cindex Sender
-Insert a new @code{Sender} header if the @code{From} header looks odd.
-@item sendsys
-@cindex sendsys
-Check for the existence of version and sendsys commands.
-@item shoot
-Check whether the domain part of the @code{Message-ID} header looks ok.
-@item shorten-followup-to
-Check whether to add a @code{Followup-To} header to shorten the number
-of groups to post to.
-@item signature
-Check the length of the signature.
-@item size
-Check for excessive size.
-@item subject
-Check whether the @code{Subject} header exists and is not empty.
-@item subject-cmsg
-Check the subject for commands.
-@item valid-newsgroups
-Check whether the @code{Newsgroups} and @code{Followup-To} headers
-are valid syntactically.
-@end table
-
-All these conditions are checked by default, except for @code{sender}
-for which the check is disabled by default if
-@code{message-insert-canlock} is non-@code{nil} (@pxref{Canceling News}).
-
 @item message-ignored-news-headers
 @vindex message-ignored-news-headers
 Regexp of headers to be removed before posting.  The default is@*

lisp/gnus/message.el

@@ -187,22 +187,26 @@ Otherwise, most addresses look like `angles', but they look like
 
 (defcustom message-syntax-checks
   (if message-insert-canlock '((sender . disabled)) nil)
-  ;; Guess this one shouldn't be easy to customize...
   "Controls what syntax checks should not be performed on outgoing posts.
 To disable checking of long signatures, for instance, add
  `(signature . disabled)' to this list.
 
 Don't touch this variable unless you really know what you're doing.
 
-Checks include `approved', `bogus-recipient', `continuation-headers',
-`control-chars', `empty', `existing-newsgroups', `from', `illegible-text',
-`invisible-text', `long-header-lines', `long-lines', `message-id',
-`multiple-headers', `new-text', `newsgroups', `quoting-style',
-`repeated-newsgroups', `reply-to', `sender', `sendsys', `shoot',
-`shorten-followup-to', `signature', `size', `subject', `subject-cmsg'
-and `valid-newsgroups'."
-  :group 'message-news
-  :type '(repeat sexp))			; Fixme: improve this
+See the Message manual for the meanings of the valid syntax check
+types."
+  :group 'message-headers
+  :link '(custom-manual "(message)Message Headers")
+  :type '(alist
+	  :key-type symbol
+	  :value-type (const disabled)
+	  :options (approved bogus-recipient continuation-headers
+		    control-chars empty existing-newsgroups from illegible-text
+		    invisible-text long-header-lines long-lines message-id
+		    multiple-headers new-text newgroups quoting-style
+		    repeated-newsgroups reply-to sender sendsys shoot
+		    shorten-followup-to signature size subject subject-cmsg
+		    valid-newsgroups)))
 
 (defcustom message-required-headers '((optional . References)
 				      From)