commit-tree: document -S option consistently

Commit ba3c69a (commit: teach --gpg-sign option, 2011-10-05) added the
-S option but documented it in the command usage without indicating that
the value is optional and forgot to mention it in the manpage.  Later
commit 098bbdc (Add -S, --gpg-sign option to manpage of "git commit",
2012-10-21) documented the option in the porcelain manpage.

Use wording from the porcelain manpage to document the option in the
plumbing manpage.  Also update the commit-tree usage summary to indicate
that the -S value is optional to be consistent with the manpage and with
the implementation.

Signed-off-by: Brad King <brad.king@kitware.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-commit-tree.txt

@@ -10,7 +10,9 @@ SYNOPSIS
 --------
 [verse]
 'git commit-tree' <tree> [(-p <parent>)...] < changelog
-'git commit-tree' [(-p <parent>)...] [(-m <message>)...] [(-F <file>)...] <tree>
+'git commit-tree' [(-p <parent>)...] [-S[<keyid>]] [(-m <message>)...]
+		  [(-F <file>)...] <tree>
+
 
 DESCRIPTION
 -----------
@@ -52,6 +54,9 @@ OPTIONS
 	Read the commit log message from the given file. Use `-` to read
 	from the standard input.
 
+-S[<keyid>]::
+	GPG-sign commit.
+
 
 Commit Information
 ------------------

builtin/commit-tree.c

@@ -10,7 +10,7 @@
 #include "utf8.h"
 #include "gpg-interface.h"
 
-static const char commit_tree_usage[] = "git commit-tree [(-p <sha1>)...] [-S<signer>] [-m <message>] [-F <file>] <sha1> <changelog";
+static const char commit_tree_usage[] = "git commit-tree [(-p <sha1>)...] [-S[<keyid>]] [-m <message>] [-F <file>] <sha1> <changelog";
 
 static void new_parent(struct commit *parent, struct commit_list **parents_p)
 {