git-add.txt: document 'interactive.singlekey'

This is documented in the section about the 'Interactive Mode', rather than for
the option '--patch', since this is the section is where people go to learn
about '--patch'.

Helped-by: Jeff King <peff@peff.net>
Mentored-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Valentin Haenel <valentin.haenel@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-add.txt

@@ -272,7 +272,8 @@ patch::
   This lets you choose one path out of a 'status' like selection.
   After choosing the path, it presents the diff between the index
   and the working tree file and asks you if you want to stage
-  the change of each hunk.  You can say:
+  the change of each hunk.  You can select one of the following
+  options and type return:
 
        y - stage this hunk
        n - do not stage this hunk
@@ -291,6 +292,9 @@ patch::
 +
 After deciding the fate for all hunks, if there is any hunk
 that was chosen, the index is updated with the selected hunks.
++
+You can omit having to type return here, by setting the configuration
+variable `interactive.singlekey` to `true`.
 
 diff::