bug#43853: message-insert-signature should lead to full docs

["積丹尼 Dan Jacobson" <jidanni@jidanni.org>]

!C-c C-w runs the command message-insert-signature (found in
!message-mode-map), which is an interactive compiled Lisp function in
!‘message.el’.
!
!It is bound to C-c C-w, <menu-bar> <Message> <Insert Signature>.
!
!(message-insert-signature &optional FORCE)
!
!Insert a signature.  See documentation for variable ‘message-signature’.

OK

!message-signature is a variable defined in ‘message.el’.
!Its value is nil
!Original value was t
!
!  This variable may be risky if used as a file-local variable.
!
!Documentation:
!String to be inserted at the end of the message buffer.
!If t, 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.
!
!You can customize this variable.
!
!This variable was introduced, or its default value was changed, in
!version 23.2 of Emacs.

OK

but all that never gets around to mentioning e.g.,

(defcustom message-signature-insert-empty-line t
  "If non-nil, insert an empty line before the signature separator."

etc.

So the C-c C-w user never finds out how to get rid of that newline
without looking at the source code, (or (info "(message) Insertion
Variables".))
emacs-version "26.3"