bug#28736: 24.5; doc of `push'

[Eli Zaretskii]

> Date: Sat, 7 Oct 2017 19:56:58 -0700 (PDT)
> From: Drew Adams <drew.adams@oracle.com>
> 
> Both the doc string and the Elisp manual entry should say that `push'
> returns the new value of the place that it updates.  The doc says
> nothing about the return value.

That usually means we don't want to advertise the return value, and
that programs should not depend on it.  Why is that a problem in this
case?