Documentation: document am --no-gpg-sign

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

Documentation/git-am.txt

@@ -148,9 +148,12 @@ default.   You can use `--no-utf8` to override this.
 
 -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.
+	stuck to the option without a space. `--no-gpg-sign` is useful to
+	countermand both `commit.gpgSign` configuration variable, and
+	earlier `--gpg-sign`.
 
 --continue::
 -r::