Doc: NR 2.4.1 - add Hammer/Pull snippets (issue 46730043)

[tdanielsmusic]

LGTM, apart from a couple of nits


https://codereview.appspot.com/46730043/diff/1/Documentation/snippets/new/hammer-on-and-pull-off-using-chords.ly
File Documentation/snippets/new/hammer-on-and-pull-off-using-chords.ly
(right):

https://codereview.appspot.com/46730043/diff/1/Documentation/snippets/new/hammer-on-and-pull-off-using-chords.ly#newcode8
Documentation/snippets/new/hammer-on-and-pull-off-using-chords.ly:8: is
drawn. However @q{double arcs} are possible by using the
by setting the

https://codereview.appspot.com/46730043/diff/1/Documentation/snippets/new/hammer-on-and-pull-off-using-chords.ly#newcode9
Documentation/snippets/new/hammer-on-and-pull-off-using-chords.ly:9:
@code{doubleSlur} property to true.
doubleSlurs (with an "s")

https://codereview.appspot.com/46730043/