Merge branch 'la/doc-choose-starting-point-fixup'

Clarify how to pick a starting point for a new topic in the SubmittingPatches document. * la/doc-choose-starting-point-fixup: SubmittingPatches: use of older maintenance tracks is an exception SubmittingPatches: explain why 'next' and above are inappropriate base SubmittingPatches: choice of base for fixing an older maintenance track