Merge branch 'po/range-doc' into maint

Merge branch 'po/range-doc' into maint

Clarify various ways to specify the "revision ranges" in the
documentation.

* po/range-doc:
  doc: revisions: sort examples and fix alignment of the unchanged
  doc: revisions: show revision expansion in examples
  doc: revisions - clarify reachability examples
  doc: revisions - define `reachable`
  doc: gitrevisions - clarify 'latter case' is revision walk
  doc: gitrevisions - use 'reachable' in page description
  doc: revisions: single vs multi-parent notation comparison
  doc: revisions: extra clarification of <rev>^! notation effects
  doc: revisions: give headings for the two and three dot notations
  doc: show the actual left, right, and boundary marks
  doc: revisions - name the left and right sides
  doc: use 'symmetric difference' consistently