Add additional in-line and NatSpec comments

[mgcolburn]

Add additional in-line and NatSpec comments

Type: Code Quality
Target: Throughout

Description

Solidity is often annotated using NatSpec comments (https://solidity.readthedocs.io/en/v0.5.11/natspec-format.html). There are a lot of optional tags you can include or not as you see fit, but it could make for a nice place to start for improving code documentation. The function descriptions in the accompanying README on Github could almost be copied in directly. Additionally, more in-line comments would be nice to see, especially around some of the denser arithmetic in the code.