bug#69983: Use category for display-buffer-alist

[Juri Linkov]

>>> But if so, you should say that this works only
>>> when called from 'display-buffer'.
>>
>> This already says so:
> ...
>> +  * `category': the buffer matches a category as a symbol if
>> +    the caller of `display-buffer' provides `(category . symbol)'
>> +    in its action argument.
> ...
>
> I was never really fond of using 'buffer-match-p' for 'display-buffer'.
> This is an example why.  A caller of 'buffer-match-p' who does not know
> about 'display-buffer' has to care about whether the CONDITION passed to
> it contains a 'category' element.

A caller of 'buffer-match-p' does not need to care about 'category'.
Only a user who customizes 'display-buffer-alist' needs to care about 
'category'.