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