RE: master a6b5985: Avoid duplicated character classes in rx

[Drew Adams]

> > So you've never written
> > (if (setq var value) ...)
> > ?  ;-)
> 
> The return value of setq _is_ documented.

Likewise `setf'.  Which is why the return
value of `push' is _indirectly_ documented.

That indirection already commits to support
of the return value (of `push').  We should
specify it explicitly.

As does Common Lisp (even though it too
refers to `setf' in the doc).  There's no
reason not to say clearly and directly what
`push' returns.