SubmittingPatches: add more about patch descriptions

Add more information about patch descriptions.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Reviewed-by: Christoph Lameter <cl@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>