bug#43136: Mention manual insertion of message-signature-file

[積丹尼 Dan Jacobson]

On (info "(message) Insertion Variables")

‘message-signature’
     String to be inserted at the end of the message buffer.  If ‘t’
     (which is the default), the ‘message-signature-file’ file will be
     inserted instead.  If a function, the result from the function will
     be used instead.  If a form, the result from the form will be used
     instead.  If this variable is ‘nil’, no signature will be inserted
     at all.

Add:

"But you can still insert your message-signature-file by hand when
desired, using the command message-insert-signature: C-c C-w."

Gnus v5.13 GNU Emacs 26.3