Document git add -u introduced earlier.

This command was implemented, but not documented in
dfdac5d9b8.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-add.txt

@@ -7,7 +7,7 @@ git-add - Add file contents to the changeset to be committed next
 
 SYNOPSIS
 --------
-'git-add' [-n] [-v] [-f] [--interactive | -i] [--] <file>...
+'git-add' [-n] [-v] [-f] [--interactive | -i] [-u] [--] <file>...
 
 DESCRIPTION
 -----------
@@ -56,6 +56,10 @@ OPTIONS
 	Add modified contents in the working tree interactively to
 	the index.
 
+-u::
+	Update all files that git already knows about. This is what
+	"git commit -a" does in preparation for making a commit.
+
 \--::
 	This option can be used to separate command-line options from
 	the list of files, (useful when filenames might be mistaken