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

Documentation/snippets/numbering-groups-of-measures.ly

Index: Documentation/snippets/numbering-groups-of-measures.ly
diff --git a/Documentation/snippets/numbering-groups-of-measures.ly b/Documentation/snippets/numbering-groups-of-measures.ly
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+% DO NOT EDIT this file manually; it is automatically
+% generated from Documentation/snippets/new
+% Make any changes in Documentation/snippets/new/
+% and then run scripts/auxiliar/makelsr.py
+%
+% This file is in the public domain.
+%% Note: this file works from version 2.17.7
+\version "2.17.7"
+
+\header {
+  lsrtags = "contexts-and-engravers, editorial-annotations, repeats, staff-notation"
+
+  texidoc = "
+This snippet demonstrates the use of the @code{Measure_counter_engraver} to
+number groups of successive measures.  Any stretch of measures may be numbered,
+whether consisting of repetitions or not.
+
+The engraver must be added to the appropriate context.  Here, a @code{Staff}
+context is used; another possibility is a @code{Dynamics} context.
+
+The counter is begun with @code{\\startMeasureCount} and ended with
+@code{\\stopMeasureCount}.  Numbering will start by default with @code{1}, but
+this behavior may be modified by overriding the @code{count-from} property.
+
+When a measure extends across a line break, the number will appear twice, the
+second time in parentheses.
+
+"
+  doctitle = "Numbering groups of measures"
+} % begin verbatim
+
+
+\layout {
+  \context {
+    \Staff
+    \consists #Measure_counter_engraver
+  }
+}
+
+\new Staff {
+  \startMeasureCount
+  \repeat unfold 7 {
+    c'4 d' e' f'
+  }
+  \stopMeasureCount
+  \bar "||"
+  g'4 f' e' d'
+  \override Staff.MeasureCounter.count-from = #2
+  \startMeasureCount
+  \repeat unfold 5 {
+    g'4 f' e' d'
+  }
+  g'4 f'
+  \bar ""
+  \break
+  e'4 d'
+  \repeat unfold 7 {
+    g'4 f' e' d'
+  }
+  \stopMeasureCount
+}