Clean up problems with fret-diagram-terse markups

Clean up problems with fret-diagram-terse markups

  Clarify that only one indication per string can be created with
    fret-diagram-terse syntax.
  Add warning to fret-parse-terse-definition-string when a barre
    is ended on a fret without being started on that fret.

  As this does not change the allowed syntax, I do not feel a regtest
    is necessary.

The warning can be seen by executing the following code:
\version "2.19.82"

\markup {
\fret-diagram-terse "x;o;o;5-1-(;5-1;7-3-);"
}