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

[Juri Linkov]

>> >> '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.
>
> You can use "bottom" if that fits.

Currently the 'side' arg of display-buffer-in-side-window
uses such repertoire of possible values:

  left, right, top, bottom

whereas the proposed display-buffer-in-direction
for relative window positioning:

  left, right, above/up, below/down

Since 'top' and 'bottom' are precluded for relative positioning anyway,
'top' and 'bottom' are free to be used for absolute positioning:

  leftmost, rightmost, top, bottom

If the goal is to try to achieve consistency with
display-buffer-in-side-window, then it makes more sense
to think about using 'leftmost' and 'rightmost' in
display-buffer-in-side-window, but maybe there is no need
for such consistency.

Using 'bottom' and 'below' will also provide backward-compatibility of
names with display-buffer-at-bottom and display-buffer-below-selected.

There is also display-buffer--maybe-at-bottom, but I need help
from Martin because I don't understand how to replace
display-buffer--maybe-at-bottom using display-buffer-in-direction.