Make the value nil in text-quoting-style mean what it does in Emacs 25.

This is a partial reversion of yesterday's commit by the same author, which
changed the meaning of nil and introduced the new value t.

* src/doc.c (text_quoting_style, text-quoting-style)
(internal--text-quoting-flag): Revert yesterday's changes.

* lisp/cus-start.el: (top level): Amend the entry for text-quoting-style.

* etc/NEWS: Amend the entry for text-quoting-style.

* doc/lispref/control.texi (Signalling Errors)
* doc/lispref/display.texi (Displaying Messages)
* doc/lispref/strings.texi (Formatting Strings): Bind text-quoting-style to
grave rather than nil to inhibit translation of quotes.

* doc/lispref/help.texi (Keys in Documentation): Revert the description of the
proposed new default, t.

1 files changed, 16 insertions, 20 deletions

diff --git a/src/doc.c b/src/doc.c
index e2af39ef07f..3286c12675a 100644
--- a/src/doc.c
+++ b/src/doc.c
@@ -690,9 +690,8 @@ enum text_quoting_style
 text_quoting_style (void)
 {
   if (NILP (Vtext_quoting_style)
-      || (EQ (Vtext_quoting_style, Qt)
-          ? default_to_grave_quoting_style ()
-          : EQ (Vtext_quoting_style, Qgrave)))
+      ? default_to_grave_quoting_style ()
+      : EQ (Vtext_quoting_style, Qgrave))
     return GRAVE_QUOTING_STYLE;
   else if (EQ (Vtext_quoting_style, Qstraight))
     return STRAIGHT_QUOTING_STYLE;
@@ -1019,25 +1018,22 @@ syms_of_doc (void)
   Vbuild_files = Qnil;
 
   DEFVAR_LISP ("text-quoting-style", Vtext_quoting_style,
-               doc: /* How to translate single quotes in help and messages.
-Its value should be a symbol, and describes the style of quote
-substituted for ASCII quote characters GRAVE ACCENT (\\=`, \\=\\x60) and
-APOSTROPHE (\\=', \\=\\x27).  This is done in help output and in functions
-like `message' and `format-message'.  It is not done in `format'.
-
-The value nil means do not translate the quotes at all.  The value t
-(the default) acts like `curve' if curved single quotes appear to be
-displayable, and like nil otherwise.  `curve' means quote with curved
-single quotes ‘like this’.  `straight' means quote with apostrophes
-\\='like this\\='.  `grave' means do not translate quote marks and is
-now a synonym for nil.
-
-(The value t was newly introduced in Emacs 26, and in Emacs 25 nil
-meant what t means now.)  */);
-  Vtext_quoting_style = Qt;
+               doc: /* Style to use for single quotes in help and messages.
+Its value should be a symbol.  It works by substituting certain single
+quotes for grave accent and apostrophe.  This is done in help output
+and in functions like `message' and `format-message'.  It is not done
+in `format'.
+
+`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\\=';
+i.e., do not alter quote marks.  The default value nil acts like
+`curve' if curved single quotes are displayable, and like `grave'
+otherwise.  */);
+  Vtext_quoting_style = Qnil;
 
   DEFVAR_BOOL ("internal--text-quoting-flag", text_quoting_flag,
-               doc: /* If nil, a `text-quoting-style' value t is treated as `nil'.  */);
+               doc: /* If nil, a nil `text-quoting-style' is treated as `grave'.  */);
   /* Initialized by ‘main’.  */
 
   defsubr (&Sdocumentation);