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

[Juri Linkov]

>> Ok, here is a complete patch with documentation:
>
> +  * `category': the buffer matches if the caller of `display-buffer'
> +    provided `(category . symbol)' in its ACTION argument.
>
> IIUC 'buffer-match-p' may be called from anywhere.  Hence,
> mentioning 'display-buffer' here seems confusing at least.

But only 'display-buffer' provides a category.
We can't suggest to use it for other purposes.