Router Heap documentation update

[nedsels]

Two changes to Router Heap documentation:

1. u is protected and so doesn't appear in the documentation, but the descriptions of its accessors/mutators refer to it. Now, these functions' descriptions don't reference u and state explicitly what they do.
2. There are duplicate descriptions for public pure virtual functions inherited from HeapInterface and/or KAryHeap. Now, all pure virtual functions in these classes are protected, so they will only appear in the description of the child that implements them.