(Mail Format): Replace "Sender" with "Message-Id", since

(Mail Format): Replace "Sender" with "Message-Id", since
the former is not always used.
(Mail Headers): Use active voice.  Add "Mail-reply-to".
Change case of "Id".  Avoid repeated "appropriate".
(Mail Aliases): Fix previous change - use an example with a ".", so it
does actually get quoted when expanded.
(Mail Sending): Mailclient is the default on some systems.
(Citing Mail): Mention mail-indentation-spaces.
(Mail Mode Misc): Add an @dfn for "mail signature".