addition to documentation - ghost notes for drums/percussion

[bernie arai]

(append to section 7.4.3 ? or add new 7.4.4 ?)

Ghost notes

Ghost notes for drums and percussion can be created by using the
/parenthesize prefix (see 8.5.8), however by default /drummode does
not include the Parenthesis_engraver plugin which allows this.  You
must add the plugin explicitly in the context definition (see 9.2.3).
For example:

\new DrumStaff \with { \consists "Parenthesis_engraver" } <<
\context DrumVoice  = "1"  { s1 *2 }
\context DrumVoice  = "2" { s1 *2 }
\drummode {
  <<
     { hh8[ hh] <hh sn> hh16 < \parenthesize sn > hh < \parenthesize
sn > hh8 <hh sn> hh }
    \\
    { bd4 r4 bd8 bd r8 bd }
  >>
}
> >

Also note that you must add "< >" brackets around each /parenthesize statement.