Add documentation for ly:one-line-breaking. (issue 6353081)

LGTM, apart from one minor change

http://codereview.appspot.com/6353081/diff/1/Documentation/notation/spacing.itely
File Documentation/notation/spacing.itely (right):

http://codereview.appspot.com/6353081/diff/1/Documentation/notation/spacing.itely#newcode906
Documentation/notation/spacing.itely:906:  <at> ref{Minimal page breaking}.
Change . to ,

http://codereview.appspot.com/6353081/