>>> Also, we should mention the constraints used for 'lru' and that 'mru'
>>> avoids the selected window here.
>>
>> Sorry, I don't understand what constraints.  The docstring of
>> 'display-buffer-use-some-window' doesn't mention any 'lru' constraints.
>
> For historical reasons, 'display-buffer-use-some-window' avoids windows
> that are not full-width and windows on another frame via
>
>                (append alist `((lru-frames . ,frame)
>                                (window-min-width . full-width))))
>
> The default behavior can be overridden with appropriate alist entries
> which are not mentioned in the doc-string but are described in the Elisp
> manual.  Your option should mention them because they defy a "strict"
> interpretation of 'lru'.

Thanks for explanations, now these constraints are documented.

> BTW, you could have, instead of an option, provided a 'some-window'
> alist entry with the possible values 'lru', 'mru' or a function.  Why
> did you prefer the option?

I already considered adding an alist entry, but the problem is that
it's not clear where users would put it to cover display of all buffers.
But I could reconsider such solution, if you will suggest a suitable place
for users to add such an alist like (some-window . mru) for all buffers.