(Rmail Sorting): Add the keybindings associated with each

(Rmail Sorting): Add the keybindings associated with each
sort command.  Fix `rmail-sort-by-labels' name.  Mention sorting from
summary.  Mention sorts cannot be undone.
(Rmail Display): Give an example of how to use goto-address-mode.
(Rmail Editing): It's keybindings that are redefined, not commands.
Fix some typos.
(Movemail): Some minor rewording.
(Remote Mailboxes): Emacs movemail supports pop by default.
Fix some minor grammatical issues.  The "two alternative ways" to
specify a POP mailbox are really just one.  Remove all reference to the
variables rmail-pop-password and rmail-pop-password-required, obsolete
since Emacs 22.1.  Clarify the four password steps.  Emacs movemail
can support Kerberos.