commit-tree: add and document --no-gpg-sign

Document how to override commit.gpgsign configuration that is set to
true per "git commit" invocation (parse-options machinery lets us
say "--no-gpg-sign" to do so).

"git commit-tree" does not use parse-options, so manually add the
corresponding option for now.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-commit.txt

@@ -302,6 +302,10 @@ configuration variable documented in linkgit:git-config[1].
 --gpg-sign[=<keyid>]::
 	GPG-sign commit.
 
+--no-gpg-sign::
+	Countermand `commit.gpgsign` configuration variable that is
+	set to force each and every commit to be signed.
+
 \--::
 	Do not interpret any more arguments as options.