RE: document custom-variable-p

[Drew Adams]

>     `user-variable-p' is documented in the Elisp manual, but
>     `custom-variable-p' is not.
>
> In general, the internals of Custom are documented only in regard to
> how to write definitions.  Why is it important to document
> `custom-variable-p'?

In my bug report I gave this reason:

"`custom-variable-p' is apparently the predicate for variables that are
`user-variable-p' and are also customizable with Customize."

"Lisp programs should use `custom-variable-p', not `user-variable-p',
whenever they want to test whether a variable is something that a user
can customize and save."

By the latter, I meant user Lisp programs, not just internal Lisp programs -
the Elisp manual is for users who program in Emacs Lisp. A user program
might well want to distinguish customizable options from other user options.

I don't see `custom-variable-p' as being any more internal than
`user-variable-p'.