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

Clarify how to choose the starting point for a new topic in developer guidance document. * la/doc-choose-starting-point: SubmittingPatches: simplify guidance for choosing a starting point SubmittingPatches: emphasize need to communicate non-default starting points SubmittingPatches: de-emphasize branches as starting points SubmittingPatches: discuss subsystems separately from git.git SubmittingPatches: reword awkward phrasing