Redo text-quoting-style variable

Rename help-quote-translation to text-quoting-style,
and use symbols rather than characters as values.
This follows suggestions along these lines by Alan Mackenzie in:
http://lists.gnu.org/archive/html/emacs-devel/2015-06/msg00343.html
and by Drew Adams in:
http://lists.gnu.org/archive/html/emacs-devel/2015-08/msg00048.html
* doc/lispref/help.texi (Keys in Documentation)
* etc/NEWS:
* lisp/cus-start.el (standard):
* src/doc.c (Fsubstitute_command_keys, syms_of_doc):
Document and/or implement the new behavior instead of the old.
(syms_of_doc): New symbols 'grave' and 'straight'.

doc/lispref/help.texi

@@ -347,18 +347,18 @@ and @samp{\=\=} puts @samp{\=} into the output.
 @strong{Please note:} Each @samp{\} must be doubled when written in a
 string in Emacs Lisp.
 
-@defvar help-quote-translation
+@defvar text-quoting-style
 @cindex curved quotes
 @cindex curly quotes
 The value of this variable specifies the style
 @code{substitute-command-keys} uses when generating left and right
-quotes.  If the variable's value is @code{?‘} (U+2018 LEFT SINGLE
-QUOTATION MARK), the style is @t{‘like this’} with curved single
-quotes.  If the value is @code{?'} (apostrophe), the style is @t{'like
-this'} with apostrophes.  If the value is @code{?`} (grave accent),
-the style is @t{`like this'} with grave accent and apostrophe.  The
-default value @code{nil} means to use curved single quotes if
-displayable, and grave accent and apostrophe otherwise.
+quotes.  If the variable's value is @code{curve}, the style is
+@t{‘like this’} with curved single quotes.  If the value is
+@code{straight}, the style is @t{'like this'} with straight
+apostrophes.  If the value is @code{grave}, the style is @t{`like
+this'} with grave accent and apostrophe.  The default value @code{nil}
+acts like @code{curve} if curved single quotes are displayable, and
+like @code{grave} otherwise.
 @end defvar
 
 @defun substitute-command-keys string

etc/NEWS

@@ -889,14 +889,14 @@ in languages like German where downcasing rules depend on grammar.
 +++
 ** substitute-command-keys now replaces quotes.
 That is, it converts documentation strings' quoting style as per the
-value of the new custom variable ‘help-quote-translation’: ?‘ means
-use curved quotes (also known as curly quotes) ‘like this’, ?' means
-use apostrophes 'like this', ?` means use grave accent and apostrophe
-`like this', and nil (default) means use curved quotes if displayable
-and grave accent and apostrophe otherwise.  Doc strings in source code
-can use either curved quotes or grave accent and apostrophe.  As
-before, isolated apostrophes and characters preceded by \= are output
-as-is.
+value of the new custom variable ‘text-quoting-style’: ‘curve’ means
+use curved quotes (also known as curly quotes) ‘like this’, ‘straight’
+means use straight apostrophes 'like this', ‘grave’ means use grave
+accent and apostrophe `like this', and nil (default) means use curved
+quotes if displayable and grave accent and apostrophe otherwise.  Doc
+strings in source code can use either curved quotes or grave accent
+and apostrophe.  As before, isolated apostrophes and characters
+preceded by \= are output as-is.
 
 +++
 ** The character classes [:alpha:] and [:alnum:] in regular expressions
@@ -1017,8 +1017,8 @@ In the new Electric Quote mode, you can enter curved single quotes
 into documentation by typing ` and '.  Outside Electric Quote mode,
 you can enter them by typing ‘C-x 8 [’ and ‘C-x 8 ]’, or (if your Alt
 key works) by typing ‘A-[’ and ‘A-]’.  As described above under
-‘help-quote-translation’, the user can specify how to display doc
-string quotes.
+‘text-quoting-style’, the user can specify how to display doc string
+quotes.
 
 +++
 ** show-help-function's arg is converted via substitute-command-keys

lisp/cus-start.el

@@ -221,14 +221,16 @@ Leaving \"Default\" unchecked is equivalent with specifying a default of
 	     (no-redraw-on-reenter display boolean)
 
 	     ;; doc.c
-	     (help-quote-translation help
-				     (choice
-				      (character :tag "Quote with curved quotes"
-                                                 :value ?‘)
-				      (character :tag "Quote 'like this'" :value ?\')
-				      (character :tag "Quote `like this'" :value ?\`)
-				      (const :tag "Quote with curved quotes if displayable, 'like this' otherwise" nil))
-				     "25.1")
+	     (text-quoting-style
+              help
+              (choice
+               (const :tag "Quote with curved single quotes" curve)
+               (const :tag "Quote with straight apostrophes 'like this'"
+                      straight)
+               (const :tag "Quote with grave accent and apostrophe `like this'"
+                      grave)
+               (const :tag "Use curved quotes if displayable, grave accent and apostrophe otherwise" nil))
+              "25.1")
              ;; dosfns.c
 	     (dos-display-scancodes display boolean)
 	     (dos-hyper-key keyboard integer)

src/doc.c

@@ -715,7 +715,7 @@ as the keymap for future \\=\\[COMMAND] substrings.
 Each \\=‘ and \\=’ are replaced by left and right quote.  Each \\=` is
 replaced by left quote, and each ' preceded by \\=` and without
 intervening ' is replaced by right quote.  Left and right quote
-characters are specified by ‘help-quote-translation’.
+characters are specified by ‘text-quoting-style’.
 
 \\=\\= quotes the following character and is discarded; thus,
 \\=\\=\\=\\= puts \\=\\= into the output, \\=\\=\\=\\[ puts \\=\\[ into the output, and
@@ -751,11 +751,11 @@ Otherwise, return a new string.  */)
   GCPRO4 (string, tem, keymap, name);
 
   enum { unicode, grave_accent, apostrophe } quote_translation = unicode;
-  if (EQ (Vhelp_quote_translation, make_number ('`')))
+  if (EQ (Vtext_quoting_style, Qgrave))
     quote_translation = grave_accent;
-  else if (EQ (Vhelp_quote_translation, make_number ('\'')))
+  else if (EQ (Vtext_quoting_style, Qstraight))
     quote_translation = apostrophe;
-  else if (NILP (Vhelp_quote_translation)
+  else if (NILP (Vtext_quoting_style)
 	   && DISP_TABLE_P (Vstandard_display_table))
     {
       Lisp_Object dv = DISP_CHAR_VECTOR (XCHAR_TABLE (Vstandard_display_table),
@@ -1024,6 +1024,8 @@ void
 syms_of_doc (void)
 {
   DEFSYM (Qfunction_documentation, "function-documentation");
+  DEFSYM (Qgrave, "grave");
+  DEFSYM (Qstraight, "straight");
 
   DEFVAR_LISP ("internal-doc-file-name", Vdoc_file_name,
 	       doc: /* Name of file containing documentation strings of built-in symbols.  */);
@@ -1033,15 +1035,14 @@ syms_of_doc (void)
                doc: /* A list of files used to build this Emacs binary.  */);
   Vbuild_files = Qnil;
 
-  DEFVAR_LISP ("help-quote-translation", Vhelp_quote_translation,
-               doc: /* Style to use for single quotes in help.
-The value is a left single quote character of some style.
-Quote \\=‘like this\\=’ if the value is ?\\=‘ (left single quotation mark).
-Quote 'like this' if the value is ?' (apostrophe).
-Quote \\=`like this' if the value is ?\\=` (grave accent).
-The default value is nil, which means quote with left single quotation mark
-if displayable, and with grave accent otherwise.  */);
-  Vhelp_quote_translation = Qnil;
+  DEFVAR_LISP ("text-quoting-style", Vtext_quoting_style,
+               doc: /* Style to use for single quotes when generating text.
+‘curve’ means quote with curved single quotes \\=‘like this\\=’.
+‘straight’ means quote with straight apostrophes 'like this'.
+‘grave’ means quote with grave accent and apostrophe \\=`like this'.
+The default value nil acts like ‘curve’ if curved single quotes are
+displayable, and like ‘grave’ otherwise.  */);
+  Vtext_quoting_style = Qnil;
 
   defsubr (&Sdocumentation);
   defsubr (&Sdocumentation_property);