bug#3135: 23.0.92; doc of custom-variable-documentation

[Drew Adams]

> >> In my opinion, it's clear enough as it is.
> >
> > What makes it clear that VARIABLE is not necessarily a user variable
> > (`user-variable-p')?
> 
> It says nothing about VARIABLE being a special kind of variable, so it
> seems pretty obvious to me that it's talking about all kinds of
> variables. 

It might be clear to you, but not to users.  Customize is all about user
variables (options).  We should be very clear whenever we mean a variable that
is not necessarily a user variable.

What does this added clarity cost?  It's about _users_; it's not about Emacs
Dev's ease in dispatching bugs summarily.