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

[Robert Pluim <rpluim@gmail.com>]

Glenn Morris <rgm@gnu.org> writes:

> Robert Pluim wrote:
>
>> --- i/lisp/gnus/message.el
>> +++ w/lisp/gnus/message.el
>> @@ -1079,12 +1079,15 @@ message-indent-citation-function
>>  (defcustom message-signature mail-signature
>>    "String to be inserted at the end of the message buffer.
>>  If t, the `message-signature-file' file will be inserted instead.
>> +If nil, no signature is inserted.
>>  If a function, the result from the function will be used instead.
>> -If a form, the result from the form will be used instead."
>> -  :version "23.2"
>> +If a form, the result from the form will be used instead.
>
> May as well rewrite it all to use the active voice while you are there.
>

OK. I modified 'mail-signature' as well, it didnʼt describe the nil
value.

>> +See `mail-signature' for the recommended format of a signature."
>> +  :version "27.1"
>
> It's not new, and the default hasn't changed, so a version bump is not
> appropriate. It's just a doc fix.

Fixed.

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.

diff --git i/lisp/gnus/message.el w/lisp/gnus/message.el
index 66356b6fda..6ecf804e9e 100644
--- i/lisp/gnus/message.el
+++ w/lisp/gnus/message.el
@@ -1078,13 +1078,16 @@ message-indent-citation-function
 
 (defcustom message-signature mail-signature
   "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."
+If nil, no signature is inserted.
+If t, the `message-signature-file' file is inserted instead.
+If a function, the result from the function is used.
+If a form, the result from the form is used.
+See `mail-signature' for the recommended format of a signature."
   :version "23.2"
   :type '(choice string (const :tag "Contents of signature file" t)
 		 function
-		 sexp)
+		 sexp
+                 (const :tag "None" nil))
   :risky t
   :link '(custom-manual "(message)Insertion Variables")
   :group 'message-insertion)
diff --git i/lisp/mail/sendmail.el w/lisp/mail/sendmail.el
index 50dd81039e..af5270fbd9 100644
--- i/lisp/mail/sendmail.el
+++ w/lisp/mail/sendmail.el
@@ -370,6 +370,7 @@ mail-mode-map
 ;;;###autoload
 (defcustom mail-signature t
   "Text inserted at end of mail buffer when a message is initialized.
+If nil, no signature is inserted.
 If t, it means to insert the contents of the file `mail-signature-file'.
 If a string, that string is inserted.
  (To make a proper signature, the string should begin with \\n\\n-- \\n,