docs: Document interaction Fixes tag and stable branches.

For the next time I forget.

Signed-off-by: Bas Nieuwenhuizen <basni@google.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>

docs/submittingpatches.html

@@ -266,6 +266,10 @@ Note: by removing the tag [as the commit is pushed] the patch is
 Thus, drop the line <strong>only</strong> if you want to cancel the nomination.
 </p>
 
+Alternatively, if one uses the "Fixes" tag as described in the "Patch formatting"
+section, it nominates a commit for all active stable branches that include the
+commit that is referred to.
+
 <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