Document Message mode as the default mail mode.

Document Message mode as the default mail mode.

* sending.texi (Sending Mail): Document the fact that Message mode is
now the default mail mode.
(Mail Format, Mail Headers): Document mail-from-style changes.
(Mail Commands): Rename from Mail mode.  Document Message mode.
(Mail Misc): Rename from Mail mode Misc.
(Mail Sending, Header Editing, Mail Misc): Switch to Message mode
command names and update keybindings.
(Header Editing): Document message-tab.  De-document
mail-self-blind, mail-default-reply-to, and mail-archive-file-name in
favor of mail-default-headers.  Ad index entries for user-full-name and
user-mail-address.
(Citing Mail): Update changes in Message mode behavior.  Document
mail-yank-prefix.
(Mail Signature): New node, moved from Mail Misc.
(Mail Aliases): Mail abbrevs are the default with Message mode.
(Mail Methods): Note that Message mode is now the default.

* rmail.texi (Rmail Reply):
* text.texi (Text Mode):
* major.texi (Major Modes):
* mule.texi (Output Coding): Refer to Message mode.

* custom.texi (Init Examples): Add xref to Mail Header.

* emacs.texi (Top):  Fix xrefs.