SubmittingPatches: clarify DCO is our --signoff rule

The description on sign-off and DCO was written back in the days
where there was only a choice between "use sign-off and it means the
contributor agrees to the Linux-kernel style DCO" and "not using
sign-off at all will make your patch unusable".  These days, we are
trying to clarify that the exact meaning of a sign-off varies
project to project.

Let's be more explicit when presenting what _our_ rules are.  It is
of secondary importance that it originally came from the kernel
project, so move the description as a historical note at the end,
while cautioning that what a sign-off means to us may be different from
what it means to other projects contributors may have been used to.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Reviewed-by: Bradley M. Kuhn <bkuhn@sfconservancy.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/SubmittingPatches

@@ -300,15 +300,12 @@ patch.
 [[sign-off]]
 === Certify your work by adding your "Signed-off-by: " line
 
-To improve tracking of who did what, we've borrowed the
-"sign-off" procedure from the Linux kernel project on patches
-that are being emailed around.  Although core Git is a lot
-smaller project it is a good discipline to follow it.
+To improve tracking of who did what, we ask you to certify that you
+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.
 
-The sign-off is a simple line at the end of the explanation for
-the patch, which certifies that you wrote it or otherwise have
-the right to pass it on as an open-source patch.  The rules are
-pretty simple: if you can certify the below D-C-O:
+If you can certify the below D-C-O:
 
 [[dco]]
 .Developer's Certificate of Origin 1.1
@@ -338,14 +335,15 @@ d. I understand and agree that this project and the contribution
    this project or the open source license(s) involved.
 ____
 
-then you just add a line saying
+you add a "Signed-off-by" trailer to your commit, that looks like
+this:
 
 ....
 	Signed-off-by: Random J Developer <random@developer.example.org>
 ....
 
-This line can be automatically added by Git if you run the git-commit
-command with the -s option.
+This line can be added by Git if you run the git-commit command with
+the -s option.
 
 Notice that you can place your own Signed-off-by: line when
 forwarding somebody else's patch with the above rules for
@@ -353,6 +351,11 @@ D-C-O.  Indeed you are encouraged to do so.  Do not forget to
 place an in-body "From: " line at the beginning to properly attribute
 the change to its true author (see (2) above).
 
+This procedure originally came from the Linux kernel project, so our
+rule is quite similar to theirs, but what exactly it means to sign-off
+your patch differs from project to project, so it may be different
+from that of the project you are accustomed to.
+
 [[real-name]]
 Also notice that a real name is used in the Signed-off-by: line. Please
 don't hide your real name.