Add alias erc-buffer-do for erc-buffer-filter

Add alias erc-buffer-do for erc-buffer-filter

* lisp/erc/erc-goodies.el (erc-scrolltobottom-enable,
erc-scrolltobottom-mode): Prefer `erc-buffer-do' to
`erc-buffer-filter'.
(erc-move-to-prompt-mode, erc-move-to-prompt-enable): Prefer
`erc-buffer-do' to `erc-buffer-filter'.
* lisp/erc/erc-imenu.el (erc-imenu-mode, erc-imenu-enable): Prefer
`erc-buffer-do' to `erc-buffer-filter'.
* lisp/erc/erc-match.el (erc-match-enable, erc-match-mode): Prefer
`erc-buffer-do' to `erc-buffer-filter'.
* lisp/erc/erc-stamp.el (erc-stamp-mode, erc-stamp-enable): Prefer
`erc-buffer-do' to `erc-buffer-filter'.
* lisp/erc/erc.el (erc-buffer-filter): Improve doc string.
(erc-buffer-do): Add alias for new code to prefer when calling
`erc-buffer-filter' for effect.  Do this because continually having to
refer back to the doc strings and implementations of the latter as
well as `erc-buffer-p', `erc-buffer-list', and co. is unproductive.
(erc-buffer-list): Use `always' as fallback predicate.