Issue 2924: Doc: \startMeasureCount and \stopMeasureCount needs documenting (issue 7678047)

[david . nalesnik]

Reviewers: ,

Message:
Please review.

Thanks!
David

Description:
Issue 2924: Doc: \startMeasureCount and \stopMeasureCount needs
documenting

The following patch is a preliminary step towards documenting the
Measure_counter_engraver added in version 2.17.7.  The patch consists of
two snippets, the first illustrating basic usage, and the second
demonstrating
a workaround for creating centered bar numbers.

Please review this at https://codereview.appspot.com/7678047/

Affected files:
  A Documentation/snippets/centered-measure-numbers.ly
  M Documentation/snippets/contexts-and-engravers.snippet-list
  M Documentation/snippets/editorial-annotations.snippet-list
  A Documentation/snippets/new/centered-measure-numbers.ly
  A Documentation/snippets/new/numbering-groups-of-measures.ly
  A Documentation/snippets/numbering-groups-of-measures.ly
  M Documentation/snippets/repeats.snippet-list
  M Documentation/snippets/staff-notation.snippet-list