change staff produces long stems

[Graham Percival]

http://code.google.com/p/lilypond/issues/detail?id=1612

My reading of the situation is this:
1. we should turn off beam collisions for cross-staff stems
2. we should document that such beam collisions are turned off, as:
   - a @knownissue in the cross-staff stem node
   - in the changes.tely
3. the existing @knownissues patch is appreciated, but will therefore
no longer be valid


My impression is that #1 would be a 10-line patch or less.  Am I
missing something?  Can somebody make a patch for #1?

Once that's in, I'll get #2 done in a few minutes, and this issue can be closed.

Cheers,
- Graham