Doc: NR 1.2.1 Ties - added @knownissue (issue 5453066)

[k-ohara5a5a]

Looks good.
For consistency with NR5.4.6 Painting objects white, you should subtract
4 from each of the layer numbers, and remove the override of StaffSymbol
layer.

Some people have probably used white-out on text or dynamics (in their
default layer=1) to place it over the StaffSymbol, so they depend on
StaffSymbol remaining in layer 0.

(I was wondering about this when I commented on the tracker issue, but
couldn't find the reference in the manual at the time.)

Consider moving your example from 'known issues' to the main text, maybe
around line 543.

http://codereview.appspot.com/5453066/