Document the dangers of co-existing elements.

Add a potent example (and simply remedy) of how element authors can
unwittingly conflict with each other in ways that it is almost
impossible for us to guard against with low level policy/mechanism in
di-b. Fixes bug 1181309.

Change-Id: I51e845065702911371e448521327143495c3b405
Merge "Document the dangers of co-existing elements."