bug#21728: 25.0.50; [patch] mention focus-{in,out}-hook in manual

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: Aaron Ecay <aaronecay@gmail.com>,  21728-done@debbugs.gnu.org
> Date: Tue, 23 Feb 2016 16:31:47 +1100
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > No, they are also mentioned in the node "Input Focus".  And the node
> > "Focus Events", where you suggested the addition, has a
> > cross-reference to "Input Focus", near its beginning.  So perhaps what
> > we need is a sentence near that cross-reference that somehow mentions
> > these two hooks, and all the rest is already in place?  Would you like
> > to suggest an alternative patch along these lines?
> 
> Hm...  is that really necessary, though?  With that xref it seems pretty
> clear that the user should go to that node for more info on everything
> connected with input focus, I think...

The xref doesn't mention the hooks.  I suggested to add a single
sentence that does, that's all.