Documentation: Clarify '--signoff' for git-commit

'--signoff' uses commiter name always to add the signoff line,
make it explicit in the documentation.

Signed-off-by: Abhijit Bhopatkar <bain@devslashzero.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-commit.txt

@@ -92,7 +92,8 @@ OPTIONS
 
 -s::
 --signoff::
-	Add Signed-off-by line at the end of the commit message.
+	Add Signed-off-by line by the commiter at the end of the commit
+	log message.
 
 -n::
 --no-verify::