stable_kernel_rules: reorganize and update submission options

The current organization of Documentation/stable_kernel_rules.txt
doesn't clearly differentiate the mutually exclusive options for
submission to the -stable review process. As I understand it, patches
are not actually required to be mailed directly to
stable@vger.kernel.org, but the instructions do not make this clear.

Also, there are some established processes that are not listed --
specifically, what I call Option 2 below.

This patch updates and reorganizes a bit, to make things clearer.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>