bug#10770: info-display-manual replaces existing *info* buffer

[Eli Zaretskii]

> From: Glenn Morris <rgm@gnu.org>
> Cc: 10770@debbugs.gnu.org
> Date: Thu, 09 Feb 2012 14:19:33 -0500
> 
> But completion is smart now! :)
> 
> C-x b <emTAB
> 
> will happily complete to *info*<emacs>

But for that, I need to _think_ first.  When I go to another manual, I
don't want to think about how to set up the completion thingy.