Improve the optional translation of quotes

Fix several problems with the recently-added custom variable
help-quote-translation where the code would quote inconsistently
in help buffers.  Add support for quoting 'like this', which
is common in other GNU programs in ASCII environments.  Change
help-quote-translation to use more mnemonic values: values are now the
initial quoting char, e.g., (setq help-quote-translation ?`) gets the
traditional Emacs help-buffer quoting style `like this'.  Change the
default behavior of substitute-command-keys to match what's done in
set-locale-environment, i.e., quote ‘like this’ if displayable,
'like this' otherwise.
* doc/lispref/help.texi (Keys in Documentation): Document
new behavior of substitute-command-keys, and document
help-quote-translation.
* doc/lispref/tips.texi (Documentation Tips):
Mention the effect of help-quote-translation.
* etc/NEWS: Mention new behavior of substitute-command-keys,
and merge help-quote-translation news into it.
When talking about doc strings, mention new ways to type quotes.
* lisp/cedet/mode-local.el (overload-docstring-extension):
Revert my recent change to this function, which shouldn't be
needed as the result is a doc string.
* lisp/cedet/mode-local.el (mode-local-print-binding)
(mode-local-describe-bindings-2):
* lisp/cedet/srecode/srt-mode.el (srecode-macro-help):
* lisp/cus-theme.el (describe-theme-1):
* lisp/descr-text.el (describe-text-properties-1, describe-char):
* lisp/emacs-lisp/cl-generic.el (cl--generic-describe):
* lisp/emacs-lisp/eieio-opt.el (eieio-help-class)
(eieio-help-constructor):
* lisp/emacs-lisp/package.el (describe-package-1):
* lisp/faces.el (describe-face):
* lisp/help-fns.el (help-fns--key-bindings)
(help-fns--compiler-macro, help-fns--parent-mode)
(help-fns--obsolete, help-fns--interactive-only)
(describe-function-1, describe-variable):
* lisp/help.el (describe-mode):
Use substitute-command-keys to ensure a more-consistent quoting
style in help buffers.
* lisp/cus-start.el (standard):
Document new help-quote-translation behavior.
* lisp/emacs-lisp/lisp-mode.el (lisp-fdefs):
* lisp/help-mode.el (help-xref-symbol-regexp, help-xref-info-regexp)
(help-xref-url-regexp):
* lisp/international/mule-cmds.el (help-xref-mule-regexp-template):
* lisp/wid-edit.el (widget-documentation-link-regexp):
Also match 'foo', in case we're in a help buffer generated when
help-quote-translation is ?'.
* src/doc.c: Include disptab.h, for DISP_CHAR_VECTOR.
(LEFT_SINGLE_QUOTATION_MARK, uLSQM0, uLSQM1, uLSQM2, uRSQM0)
(uRSQM1, uRSQM2, LSQM, RSQM): New constants.
(Fsubstitute_command_keys): Document and implement new behavior.
(Vhelp_quote_translation): Document new behavior.

2 files changed, 32 insertions, 5 deletions

diff --git a/doc/lispref/help.texi b/doc/lispref/help.texi
index ce29f3f5bc3..44a680c4a22 100644
--- a/doc/lispref/help.texi
+++ b/doc/lispref/help.texi
@@ -319,10 +319,22 @@ specifies @var{mapvar}'s value as the keymap for any following
 @samp{\[@var{command}]} sequences in this documentation string.
 
 @item `
-(grave accent) stands for a left single quotation mark (@samp{‘}).
+(grave accent) stands for a left quote, and alters the interpretation
+of the next unmatched apostrophe.
 
 @item '
-(apostrophe) stands for a right single quotation mark (@samp{’}) if
+(apostrophe) stands for a right quote if preceded by grave accent and
+there are no intervening apostrophes.  Otherwise, apostrophe stands
+for itself.
+
+@item ‘
+(left single quotation mark) stands for a left quote.
+
+@item ’
+(right single quotation mark) stands for a right quote.
+
+@item '
+(apostrophe) stands for a right quote if
 preceded by grave accent and there are no intervening apostrophes.
 Otherwise, apostrophe stands for itself.
 
@@ -335,6 +347,19 @@ 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
+@cindex curved 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 apostrophes otherwise.
+@end defvar
+
 @defun substitute-command-keys string
 This function scans @var{string} for the above special sequences and
 replaces them by what they stand for, returning the result as a string.
diff --git a/doc/lispref/tips.texi b/doc/lispref/tips.texi
index 9144497d5f1..7107bb45872 100644
--- a/doc/lispref/tips.texi
+++ b/doc/lispref/tips.texi
@@ -671,9 +671,11 @@ Documentation strings can also use an older single-quoting convention,
 which quotes symbols with grave accent @t{`} and apostrophe
 @t{'}: @t{`like-this'} rather than @t{‘like-this’}.  This
 older convention was designed for now-obsolete displays in which grave
-accent and apostrophe were mirror images.  Documentation in this older
-convention is converted to the standard convention when it is copied
-into a help buffer.  @xref{Keys in Documentation}.
+accent and apostrophe were mirror images.
+
+Documentation using either convention is converted to the user's
+preferred format when it is copied into a help buffer.  @xref{Keys in
+Documentation}.
 
 @cindex hyperlinks in documentation strings
 Help mode automatically creates a hyperlink when a documentation string