bug#4707: 23.1; Elisp manual: index constant variables for defconst

[Drew Adams]

emacs -Q
 
In the Elisp manual, `constant' is indexed for variables only for node
`Constant Variables', which is really about keywords, nil, and t.
 
I find no index entry for defconst in this regard, yet it is the way
you define constant variables.
 
Also, the node Constant Variables should mention defconst variables,
and point out how they differ from the other constant variables, which
are currently described in the node: Trying to set a defconst variable
does not raise a `setting-constant' error. Trying to set a keyword, t,
or nil does raise that error.
 
IOW, there are really two different kinds of variables that we often
call "constants" in Emacs Lisp. Some of them are truly constants
(raise an error if you try to set them), some of them are only
intended to be used as constants, with no check wrt setting them.
 
This needs to be better documented and better indexed.
 
 
 
In GNU Emacs 23.1.1 (i386-mingw-nt5.1.2600)
 of 2009-07-29 on SOFT-MJASON
Windowing system distributor `Microsoft Corp.', version 5.1.2600
configured using `configure --with-gcc (4.4)'