bug#15717: 24.3.50; document `custom-available-themes', `custom-known-themes' in Elisp manual

[Chong Yidong]

Drew Adams <drew.adams@oracle.com> writes:

> These functions are as important for programmers as is
> `custom-theme-p'.  It is especially important to distinguish them, in
> particular wrt `custom-theme-p', which uses only
> `custom-known-themes'.  An "available" theme is not necessarily a
> "known" theme.  This can be confusing.

I've added custom-known-themes to the manual, and updated the docstring
of custom-available-themes to emphasize that it's meant for things like
completion.  (I don't think custom-available-themes is suitable for the
Lisp manual, but custom-theme-load-path is already documented.)