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, 6 insertions, 3 deletions

diff --git a/src/font.c b/src/font.c
index 51625b49fa8..f7cebdce78c 100644
--- a/src/font.c
+++ b/src/font.c
@@ -5421,19 +5421,22 @@ gets the repertory information by an opened font and ENCODING.  */);
                doc: /*  Vector of valid font weight values.
 Each element has the form:
     [NUMERIC-VALUE SYMBOLIC-NAME ALIAS-NAME ...]
-NUMERIC-VALUE is an integer, and SYMBOLIC-NAME and ALIAS-NAME are symbols. */);
+NUMERIC-VALUE is an integer, and SYMBOLIC-NAME and ALIAS-NAME are symbols.
+This variable cannot be set; trying to do so will signal an error.  */);
   Vfont_weight_table = BUILD_STYLE_TABLE (weight_table);
   make_symbol_constant (intern_c_string ("font-weight-table"));
 
   DEFVAR_LISP_NOPRO ("font-slant-table", Vfont_slant_table,
                doc: /*  Vector of font slant symbols vs the corresponding numeric values.
-See `font-weight-table' for the format of the vector. */);
+See `font-weight-table' for the format of the vector.
+This variable cannot be set; trying to do so will signal an error.  */);
   Vfont_slant_table = BUILD_STYLE_TABLE (slant_table);
   make_symbol_constant (intern_c_string ("font-slant-table"));
 
   DEFVAR_LISP_NOPRO ("font-width-table", Vfont_width_table,
                doc: /*  Alist of font width symbols vs the corresponding numeric values.
-See `font-weight-table' for the format of the vector. */);
+See `font-weight-table' for the format of the vector.
+This variable cannot be set; trying to do so will signal an error.  */);
   Vfont_width_table = BUILD_STYLE_TABLE (width_table);
   make_symbol_constant (intern_c_string ("font-width-table"));