* auth-source.el (auth-source-forget-user-or-password): Clarify docs.

* auth-source.el (auth-source-forget-user-or-password): Clarify docs.
(auth-source-forget-all-cached): New convenience function.
(auth-source-user-or-password): Accept list of modes or a single mode.

* mail-source.el (mail-source-bind, mail-source-set-1): Use list of
auth-source modes.

* netrc.el (netrc-machine-user-or-password): Use list of
auth-source modes.

* nnimap.el (nnimap-open-connection): Use list of
auth-source modes.

* nntp.el (nntp-send-authinfo): Use list of
auth-source modes.