Documentation: -stable rules: upstream commit ID requirement reworded

It is a hard requirement to include the upstream commit ID in the
changelog of a -stable submission, not just a courtesy to the stable
team.  This concerns only mail submission though, which is no longer
the only way into stable.  (Also, fix a double "the".)

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Acked-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>