Documentation: reword commit --no-gpg-sign

Merge with --gpg-sign option, and clarify that --no-gpg-sign also
override earlier --gpg-sign.

Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-commit.txt

@@ -348,13 +348,12 @@ changes to tracked files.
 
 -S[<keyid>]::
 --gpg-sign[=<keyid>]::
+--no-gpg-sign::
 	GPG-sign commits. The `keyid` argument is optional and
 	defaults to the committer identity; if specified, it must be
-	stuck to the option without a space.
-
---no-gpg-sign::
-	Countermand `commit.gpgSign` configuration variable that is
-	set to force each and every commit to be signed.
+	stuck to the option without a space. `--no-gpg-sign` is useful to
+	countermand both `commit.gpgSign` configuration variable, and
+	earlier `--gpg-sign`.
 
 \--::
 	Do not interpret any more arguments as options.