Docs: SubmittingPatches: update follow-through instructions

SubmittingPatches was written in the "keep sending to Linus until something
shows up in a release" era.  Given that we don't do things that way anymore
and the system is far less lossy, update this information and add some
hints on responding to reviewer comments.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>