bug#16271: warn about quoted const's in defcustom

[Drew Adams <drew.adams@oracle.com>]

> > The argument of a `const' (or `other') inside a defcustom type should
> > (almost always) not be quoted. Yet people keep doing it.
> > It would be nice if there was some way to warn about this.
> 
> Yes, that would be nice, so I've now implemented it.  However, there's
> one false positive:
> 
> (defcustom erc-button-alist
>   '(('nicknames 0 erc-button-buttonize-nicks erc-nick-popup 0)
> ...
>       (choice :tag "Matches"
>               regexp
>               (variable :tag "Variable containing regexp")
>               ;; FIXME It really does mean 'nicknames
>               ;; rather than just nicknames.
>               (const :tag "Nicknames" 'nicknames))
> 
> And this really means 'nicknames.  Anybody got any suggestions how to
> work around that?

Don't work around it.
Don't try to warn not to quote.

In (elisp) `Composite Types', (1) mention that
you typically do not want to quote, and (2)
show an example of the exception, with a quote,
saying what the behavior is in that case.

Warnings are not appropriate here, and will
likely confuse.  Making the case clear in the
doc is helpful.