small documentation update

[David Whipple]

http://lilypond.org/doc/v2.8/Documentation/user/lilypond/Line-breaking.html#Line-breaking

This page lists a bug stating:

Line breaks can only occur if there is a “proper” bar line.
 A note which is hanging over a bar line is not 
proper, such as

          c4 c2 c2 \break   % this does nothing
     c2 c4 |           % a break here would work
     c4 c2 c4 ~ \break % as does this break
     c4 c2 c4

However, your regression tests lists a \set Score.allowBeamBreak = ##t 
that overrides this behavior.  So this is no longer a bug.

Just a little FYI