Fix and improve documentation of mail-related features

Fix and improve documentation of mail-related features

* lisp/simple.el (compose-mail): Clarify the effect of the
CONTINUE argument.
* lisp/mail/sendmail.el (mail-from-style): Update the RFC value in
the obsolescence warning text.

* doc/emacs/sending.texi (Sending Mail): Fix the description of
the behavior of 'C-x m' wrt prefix argument.
(Mail Headers): Remove the description of 'mail-from-style'.

* etc/NEWS: Mention that 'mail-from-style' is obsolete.

(Bug#40561)