Doc: NR addition to existsing @knownissue

Doc: NR addition to existsing @knownissue

As per Mike Solomon's suggestion, added additiona additional sentence
 to exitsing @knownissue for collisions with automatic beaming.

Included a simple @lilypond to illustrate the issue (thanks Mike!)

Also took opportunity to tidy up the cindex for beams (make them more
consistent - beam vs beams vs beaming etc.) added a few more and removed
some repeated ones.

removed some unnecessary line spaces in the itely file itself