Doc: Tuplet-bracket

[Rune Zedeler]

The documentation says that a bracket is "optionally" added to the 
tuplet, but it does not specify how.
For some reason it is not documented in the TupletBracket internal 
documentation either (?)

The property  TupletBracket #'bracket-visibility  should be set to 
either ##t, ##f or #'if-no-beam
Graham do you add a note in the doc?

-Rune