Improve documentation of "constant" symbols

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.