Update git-add.txt according to the new possibilities of 'git add -p'.

The text is merely cut-and-pasted from git-add--interactive.perl. The
cut-and-paste also fixes a typo.
Signed-off-by: NMatthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-add.txt

@@ -245,9 +245,11 @@ patch::
 
        y - stage this hunk
        n - do not stage this hunk
-       q - quite, do not stage this hunk nor any of the remaining ones
+       q - quit, do not stage this hunk nor any of the remaining ones
        a - stage this and all the remaining hunks in the file
        d - do not stage this hunk nor any of the remaining hunks in the file
+       g - select a hunk to go to
+       / - search for a hunk matching the given regex
        j - leave this hunk undecided, see next undecided hunk
        J - leave this hunk undecided, see next hunk
        k - leave this hunk undecided, see previous undecided hunk