Issue 4078: Doc: Use variables rather than instrument definitions

Issue 4078: Doc: Use variables rather than instrument definitions

  Using variables for switching character names in vocal music
  rather than instrument definitions is easier to understand,
  easier to set up and quicker to type.  It also frees up the
  use of instrument definitions so they may be tailored for
  better use in instrumental compositions.

  Remove the mention of \transposition.  This is never required
  in vocal music.

  Adjust references to match the changes.