Re: document custom-variable-p

[Richard Stallman]

    Customize is good for interactive editing and saving of options etc. But its
    UI is fixed and somewhat limited. I expect that users (e.g. third-party
    libraries) may well want to provide commands or other UI thingies that let
    you interface with the functionalities provided by custom: editing that
    respects specified types, and possibly saving changes.

    To create such UI extensions, it would be useful to be able to recognize
    variables that are editable and savable in custom from other user variables
    (those that just have "*..." as doc string).

I see no need to borrow trouble.  If and when people undertake such
projects, I am sure that finding out about `custom-variable-p' will be
the least of their difficulties.  If and when such packages become common,
there may be a good reason to document `custom-variable-p'.