Doc addition

[Markus Schneider]

In the program reference regarding the bar-line-interface
http://lilypond.org/doc/v2.9/Documentation/user/lilypond-internals.html#bar-line-interface
there is one type of bar lines missing:

"||:"

This also applies partially to "6.4.5 Bar lines" in the current doc.
http://lilypond.org/doc/v2.9/Documentation/user/lilypond.html#Bar-lines
A visual example of this bar type would be nice there. I drafted an example
to work with - see below.

HTH
Markus

%%% Begin Snippet
\paper {
  #(define dump-extents #t)

  % line-width = 160\mm - 2.0 * 0.4\in
  ragged-right = ##t
  indent = 0\mm
}

\layout {
  \context {
    \Score
    \override BarNumber #'break-visibility = #all-invisible
  }

}



% ****************************************************************
% ly snippet:
% ****************************************************************
\sourcefilename "double-bar-repeat-bar-line.ly"

\header {

    texidoc = "Repeat bar with automatic double bar at line break"

}

\layout { ragged-right = ##t }

\relative {
    \override Score.RehearsalMark #'padding = #3

    c4 c c c
    \bar "||:" \mark \markup { \fontsize #-3 \typewriter "unbroken" \simple
#"||:" }
    c c c c
    \break
    \bar "||:" \mark \markup { \fontsize #-3 \typewriter "broken" \simple
#"||:" }
    c c c c

}
%%% End Snippet