stable: Update documentation to clarify preferred procedure

Clearly specify that "Cc: stable@vger.kernel.org" is strongly preferred so
that developers understand that the other options should only be used when
absolutely required.

Also specify how upstream commit ids should be referenced in patches
submitted directly to stable (I gathered this from looking at the stable
archives), and specify that any modified patches for stable should be
clearly documented in the patch description.

Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>