fix[N04] Add additional documentation

[chrismaree]

Problem:
Although most of the codebase is thoroughly documented, there are a few instances where
documentation is lacking. For instance:
In the HubPool contract the public unclaimedAccumulatedProtocolFees variable has no inline
documentation.
In the HubPool contract the inline documentation accompanying
PoolRebalanceLeaf.netSendAmounts, although lengthy, could benefit from additional
clarification around the case of negative values. It could clarify further that in such cases the
actual netSendAmounts value is ignored, but it should match the amountToReturn parameter in
the RelayerRefundLeaf.
Many of the functions in the MerkleLib library are missing NatSpec @return statements.

Solution: all places with missing documentation were updated.

[nicholaspai]

LGTM after committing my suggestions!