Document 23-commitment and MerklePath better

[gjermundgaraba]

Document how MerklePath should be constructed and what the values mean in different contexts. In particular, the prefix use case we should document more clearly what is expected, and when the last element should be empty. One useful comment from the audit we should incorporate is something along the lines of "the merkle path tells us the number of trees we are dealing with".

We can also consider if we want to have two separate types for MerklePrefix and MerklePath to make the distinction and use cases clearer. It might make the current context more obvious when reading code that uses these types.

In any case, I suggest we add some more, and clearer, documentation in both the go code and in the proto files.