bug#42210: Add -other-window variants of project-prefix-map commands

[Drew Adams]

1. I disagree with calling the macro `define-other-window-command'.

My disagreement is this: Defining an other-window command
should do just that.  It should not define a command that
only "prefers" to display in another window.  It should
define a command that actually displays in another window.

And your doc string in fact says the latter, though the
behavior is, I guess, only the former.  Please consider
renaming the macro and fixing the doc string, to make clear
that it's NOT other-window but only maybe-other-window.

2. Why "resultant buffer".  What does the buffer result
from?  It's about whatever FUNCTION displays, no?

3. Presumably FUNCTION must be a _command_.  If so,
the arg should be called COMMAND, and the doc adjusted
accordingly.

#2 and #3 are minor.  I'm more concerned about #1.
Emacs has many commands that are _really_ other-window.
They generally use `pop-to-buffer' or
`switch-to-buffer-other-window'.

If this macro produces commands that only "prefer" to
use another window, then the name and doc string are
false advertising.

4. Why not have a macro that _really_ provides an
other-window command?