restrainOpenStrings: please fix snippet in Documentation/notation/fretted-strings.itely

[Federico Bruni]

In the mentioned file, around line 236, the following snippet should be 
changed because of the new property introduced by Marc (issue 2348):

@lilypond[quote,ragged-right,verbatim]
\new StaffGroup <<
   \new Staff \relative c {
     \clef "treble_8"
     \time 2/4
     c16 d e f g4
     c,16\5 d\5 e\4 f\4 g4\4
     c,16 d e f g4
   }
   \new TabStaff \relative c {
     c16 d e f g4
     c,16\5 d\5 e\4 f\4 g4\4
     \set TabStaff.minimumFret = #5
     c,16 d e f g4
   }
>>
@end lilypond

Just add a line after \set TabStaff.minimumFret:

\set TabStaff.minimumFret = #5
\set TabStaff.restrainOpenStrings = ##t

And on TabStaff d will be displayed on 5th fret of 5th string, g will be 
displayed on 5th fret of 4th string, as the example would like to 
demonstrate.

Maybe it could be also a good chance to document this new property.

Please put me in CC (I don't read lilypond-bug).
Thanks,
Federico