Doc: NR Clarify finer point of repeat unfold (issue 5075047)

[janek . lilypond]

LGTM.

From what i see, the surprise in this behaviour comes from two meanings
of "music expression" - it can be understood as "a piece of ly input" or
"a piece of music".  Shall we write a sentence about this difference?
I.e. "Using \repeat unfold is equal to writing out a fragment of music
several times over, not a fragments of ly input"?

cheers,
Janek

http://codereview.appspot.com/5075047/