docs: document how to (self-) reject stable patches

Document what has been the unofficial way to self-reject stable patches.

Namely: drop the mesa-stable tag and push the commit.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Nanley Chery <nanley.g.chery@intel.com>
diff --git a/docs/submittingpatches.html b/docs/submittingpatches.html
index d1ca68d..141cec2 100644
--- a/docs/submittingpatches.html
+++ b/docs/submittingpatches.html
@@ -249,6 +249,13 @@
 patches to the mesa-dev@ mailing list. If you prefer using --suppress-cc that
 won't have any effect negative effect on the patch nomination.
 
+<p>
+Note: by removing the tag [as the commit is pushed] the patch is
+<strong>explicitly</strong> rejected from inclusion in the stable branch(es).
+<br>
+Thus, drop the line <strong>only</strong> if you want to cancel the nomination.
+</p>
+
 <h2 id="criteria">Criteria for accepting patches to the stable branch</h2>
 
 Mesa has a designated release manager for each stable branch, and the release