Fix regression involving erc-query-buffer-p

Fix regression involving erc-query-buffer-p

* lisp/erc/erc.el (erc-server-or-unjoined-channel-buffer-p): Doc.
(erc-query-buffer-p): Don't return non-nil in non-ERC buffers and
server buffers, and continue to honor string arguments.  The
regression was introduced by 3d87e343 "Use modern fallback for channel
name detection in ERC".  Thanks to Libera user mekeor for reporting
this bug.
* test/lisp/erc/erc-tests.el (erc-query-buffer-p): New test.
(Bug#67220)

(cherry picked from commit 473189ab6902b0488f8001fdf993522b82740663)