Revision 271639

JavaDoc comments and a couple of code changes.

Code changes:
Removed an unneeded nullity test null instanceof Foo is always false.
Changed replaceChild(UnknownElement, Object) with two methods:
replaceChild(UnknownElement,Task) and
replaceChild(UnknownElement,RuntimeConfigurable) to make it
absolutely clear that you can't just replace the children with random
elements.

I must admit I'm still far from entirely clear exactly how 
UnknownElement works. It probably counts as black magic...