Doc: LM Tweaking output: Fix 1180

Doc: LM Tweaking output: Fix 1180

 - Use explicitly instantiated voices (thanks Neil)

   First, it becomes a better example to show newcomers.

   It also ensures the voice contexts remain visible so collision
   avoidance works correctly, even if a break occurs at the end
   of measure 3.  However, if the overridden line-width is
   removed the break occurs after measure 2, and causes the
   phrasing slur to collide with the 6/4 time signature, even with
   explicit voice declarations.