Improve documentation of "constant" symbols

* src/font.c (syms_of_font) <font-weight-table, font-slant-table>
<font-width-table>:
* src/data.c (syms_of_data) <most-positive-fixnum>
<most-negative-fixnum>:
* src/buffer.c (syms_of_buffer) <enable-multibyte-characters>:
Mention in the doc strings that these variables are read-only.

* doc/lispref/variables.texi (Creating Buffer-Local): Document
that making a constant variable buffer-local signals an error.
* doc/lispref/variables.texi (Constant Variables):
* doc/lispref/errors.texi (Standard Errors): More accurate and
up-to-date documentation of which symbols cannot be assigned
values.

1 files changed, 4 insertions, 2 deletions

diff --git a/src/data.c b/src/data.c
index b4f6fd5c652..d54c46d72bf 100644
--- a/src/data.c
+++ b/src/data.c
@@ -3880,12 +3880,14 @@ syms_of_data (void)
   set_symbol_function (Qwholenump, XSYMBOL (Qnatnump)->u.s.function);
 
   DEFVAR_LISP ("most-positive-fixnum", Vmost_positive_fixnum,
-	       doc: /* The largest value that is representable in a Lisp integer.  */);
+	       doc: /* The largest value that is representable in a Lisp integer.
+This variable cannot be set; trying to do so will signal an error.  */);
   Vmost_positive_fixnum = make_number (MOST_POSITIVE_FIXNUM);
   make_symbol_constant (intern_c_string ("most-positive-fixnum"));
 
   DEFVAR_LISP ("most-negative-fixnum", Vmost_negative_fixnum,
-	       doc: /* The smallest value that is representable in a Lisp integer.  */);
+	       doc: /* The smallest value that is representable in a Lisp integer.
+This variable cannot be set; trying to do so will signal an error.  */);
   Vmost_negative_fixnum = make_number (MOST_NEGATIVE_FIXNUM);
   make_symbol_constant (intern_c_string ("most-negative-fixnum"));