Documentation: merge commit-tree --[no-]gpg-sign

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

Documentation/git-commit-tree.txt

@@ -61,13 +61,11 @@ OPTIONS
 
 -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::
-	Do not GPG-sign commit, to countermand a `--gpg-sign` option
-	given earlier on the command line.
+	stuck to the option without a space. `--no-gpg-sign` is useful to
+	countermand a `--gpg-sign` option given earlier on the command line.
 
 Commit Information
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