SubmittingPatches: tighten wording on "sign-off" procedure

The text says "if you can certify DCO then you add a Signed-off-by
trailer".  But it does not say anything about people who cannot or
do not want to certify.  A natural reading may be that if you do not
certify, you must not add the trailer, but it shouldn't hurt to be
overly explicit.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/SubmittingPatches

@@ -307,7 +307,7 @@ wrote the patch or have the right to pass it on under the same license
 as ours, by "signing off" your patch.  Without sign-off, we cannot
 accept your patches.
 
-If you can certify the below D-C-O:
+If (and only if) you certify the below D-C-O:
 
 [[dco]]
 .Developer's Certificate of Origin 1.1