Document and/or standardize branch length descriptors

[snacktavish]

The translation from the option sin the curator dropdown to the nexson values are a bit confusing, and I cannot locate documentation on the values.
It looks like "estimated number of changes "-> "ot:changesCount" and "Expected changes per site" -> "ot:substitutionCount". These are not super intutitive, and should be documented. Maybe here
https://github.com/OpenTreeOfLife/germinator/wiki/Studies-API-v3#properties