Cross-reference the message/error control variables

* src/keyboard.c (syms_of_keyboard): Mention set-message-function
in the command-error-function doc string...

* src/xdisp.c (syms_of_xdisp): ... and vice versa (bug#13752).

src/keyboard.c

@@ -12303,7 +12303,10 @@ Called with three arguments:
 - the error data, a list of the form (SIGNALED-CONDITION . SIGNAL-DATA)
   such as what `condition-case' would bind its variable to,
 - the context (a string which normally goes at the start of the message),
-- the Lisp function within which the error was signaled.  */);
+- the Lisp function within which the error was signaled.
+
+Also see `set-message-function' (which controls how non-error messages
+are displayed).  */);
   Vcommand_error_function = intern ("command-error-default-function");
 
   DEFVAR_LISP ("enable-disabled-menus-and-buttons",

src/xdisp.c

@@ -35639,8 +35639,10 @@ as usual.  If the function returns a string, the returned string is
 displayed in the echo area.  If this function returns any other non-nil
 value, this means that the message was already handled, and the original
 message text will not be displayed in the echo area.
-See also `clear-message-function' that can be used to clear the
-message displayed by this function.  */);
+
+Also see `clear-message-function' (which can be used to clear the
+message displayed by this function), and `command-error-function'
+(which controls how error messages are displayed).  */);
   Vset_message_function = Qnil;
 
   DEFVAR_LISP ("clear-message-function", Vclear_message_function,