docs: backporting: address feedback

This addresses a few comments/issues in my v2 submission:

- repeated word: 'run' from kernel test robot
- emacs/ediff mode from Jon Corbet
- various comments from Willy Tarreau
- more backporting advice from Ben Hutchings
- a couple more cherry-pick tips from Harshit Mogalapalli
- add a bit about stable submissions

Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Link: https://lore.kernel.org/r/20231023135722.949510-1-vegard.nossum@oracle.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>