* doc/misc/message.texi (Forwarding): Document it.
* lisp/gnus/message.el (message-forward-ignored-headers): Improve
documentation.
(message-forward-included-headers): Ditto.
(message-forward-included-mime-headers): New user option.
(message-remove-ignored-headers): Use it to preserve the necessary
MIME headers.
(message-forward-make-body): Remove headers when forwarding as
MIME, too.
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