bug#50518: Minor doc bug: pop-up-frames

[martin rudalics]

> The variable pop-up-frames is not marked obsolete nor does the
> docstring say it exists for backward compatibility -- only the
> Info manual says it.  As a result, it's an unnecessary confusing
> factor while you are exploring the display-buffer machinery.

`pop-up-frames' is semi-obsolete.  Making it obsolete would give too
many warnings - it's way too often used in our code base.

The Info manual says that "This variable is provided mainly for backward
compatibility."  It should not be used in new code because there is no
guarantee that it will work - newer functions might not pay attention to
it.

martin