bug#35592: Use display-buffer-in-direction for completion-like windows

[martin rudalics]

>>> 'bottommost' is too ugly word ;)
>>
>> There's nothing ugly about it, IMO.  We already use it in a few places
>> in the manuals.
>
> Using bottommost as superlative of bottom sounds as weird as using
> bestest instead of best.

I pushed to master the version you preferred.  Please have a look (in
particular at the documentation).

Thanks, martin