(Mail Format): Fix typo. Add index entry for mail-header-separator.

(Mail Format): Fix typo.  Add index entry for mail-header-separator.
(Mail Headers): Put info about initialization and changing in one place
at the start.  Update FCC section for mbox Rmail.  Clarify From
section, mention mail-setup-with-from.  Clarify Reply-to section.
Add Mail-followup-to and mail-mailing-lists.  Clarify References section.
(Mail Aliases): Update example, make less contentious.
Update for name change of mail-interactive-insert-alias.
(Mail Mode): Remove mention of `%' as a word separator.
(Mail Sending): Mention mail-send-hook.  Mention Mailclient.
(Header Editing): Add reply-to, mail-reply-to, and mail-followup-to commands.
Clarify FCC handling.  In mail-complete, add reference to
Mail Aliases section, and mention mail-complete-function.
(Citing Mail): Mention mail-yank-ignored-headers and mail-citation-hook.
(Mail Mode Misc): Clarify the mail-signature function.  Add basic
signature netiquette.  Explain how the mail hooks work when continuing
a composition.
(Mail Amusements): Internationalize the spook section a bit.
Remove the spook mail-setup-hook example, since it doesn't work well.
Mention fortune-file.
(Mail Methods): Mention read-mail-command.