Update Sending Mail chapter of Emacs manual.

Update Sending Mail chapter of Emacs manual.

* sending.texi (Sending Mail): Document initial mail buffer name,
and changed multiple mail buffer behavior.
(Mail Format): Put the example at the top of the section.
(Mail Headers): Move discussion of "From" to the top.
(Mail Sending): Document sendmail-query-once.
(Citing Mail): Make it less Rmail-specific.