Add -S, --gpg-sign option to manpage of "git commit"

git commit -S, --gpg-sign was mentioned in the program's help message,
but not in the manpage.

This adds an equivalent entry for the option in the manpage.
Signed-off-by: NTom Jones <tom@oxix.org>
Signed-off-by: NJeff King <peff@peff.net>

Documentation/git-commit.txt

@@ -13,7 +13,7 @@ SYNOPSIS
 	   [-F <file> | -m <msg>] [--reset-author] [--allow-empty]
 	   [--allow-empty-message] [--no-verify] [-e] [--author=<author>]
 	   [--date=<date>] [--cleanup=<mode>] [--status | --no-status]
-	   [-i | -o] [--] [<file>...]
+	   [-i | -o] [-S[<keyid>]] [--] [<file>...]
 
 DESCRIPTION
 -----------
@@ -276,6 +276,10 @@ configuration variable documented in linkgit:git-config[1].
 	commit message template when using an editor to prepare the
 	default commit message.
 
+-S[<keyid>]::
+--gpg-sign[=<keyid>]::
+	GPG-sign commit.
+
 \--::
 	Do not interpret any more arguments as options.