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, 5 insertions, 2 deletions

diff --git a/doc/lispref/errors.texi b/doc/lispref/errors.texi
index 1f67819c34e..cd22b70800d 100644
--- a/doc/lispref/errors.texi
+++ b/doc/lispref/errors.texi
@@ -172,8 +172,11 @@ The message is @samp{Search failed}.  @xref{Searching and Matching}.
 
 @item setting-constant
 The message is @samp{Attempt to set a constant symbol}.  This happens
-when attempting to assign values to @code{nil}, @code{t}, and keyword
-symbols.  @xref{Constant Variables}.
+when attempting to assign values to @code{nil}, @code{t},
+@code{most-positive-fixnum}, @code{most-negative-fixnum}, and keyword
+symbols.  It also happens when attempting to assign values to
+@code{enable-multibyte-characters} and some other symbols whose direct
+assignment is not allowed for some reason.  @xref{Constant Variables}.
 
 @c simple.el
 @item text-read-only