SubmittingPatches: use of older maintenance tracks is an exception

SubmittingPatches: use of older maintenance tracks is an exception

While we could technically fix each and every bug on top of the
commit that introduced it, it is not necessarily practical.  For
trivial and low-value bugfixes, it often is simpler and sufficient
to just fix it in the current maintenance track, leaving the bug
unfixed in the older maintenance tracks.

Demote the "use older maintenance track to fix old bugs" as a side
note, and explain that the choice is used only in exceptional cases.

Signed-off-by: Junio C Hamano <gitster@pobox.com>