stable: update documentation to ask for kernel version

It would save me an email round-trip asking which stable tree(s) that a
patch should be applied to, so document that the tree number should be
specified in the email request.

Reported-by: Tomoya MORINAGA <tomoya.rohm@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>