bug#32680: 27.0.50; Improve docstring and customization for 'message-signature'

[Lars Ingebrigtsen]

Robert Pluim <rpluim@gmail.com> writes:

> 2018-09-10  Robert Pluim  <rpluim@gmail.com>
>
>       * lisp/gnus/message.el (message-signature): Allow nil as a
>       customizable value, and describe its effect.
>       * lisp/mail/sendmail.el (mail-signature): Describe effect of nil value.

Thanks; I've now applied this to Emacs 27.1 with Glenn's tweaks to the
text.

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