doc: add default that all fixes should be backported

doc: add default that all fixes should be backported

Set the starting point that all commits on master branch
with Fixes tag should be backported to relevant stable/LTS
branches, and explain that the submitter may indicate it is
not suitable for backport.

Of course there will be exceptions that will crop up from time
to time that need discussion, so also add a sentence for that.

This is to ensure that there is consistency between what is
backported to stable/LTS branches, remove some subjectivity
as to what constitutes "a fix" and avoid possible conflicts
for future backports.

Signed-off-by: Kevin Traynor <ktraynor@redhat.com>
Acked-by: Luca Boccassi <bluca@debian.org>
Acked-by: Aaron Conole <aconole@redhat.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>