Update code comments

[jsahleen]

Update the code comments in msg so that all classes, properties, functions and constants have comments for API documentation generation.

User Stories

1. As a msg library user, I want everything to be properly commented in code, so that I can generate and view API documentation.
   • Classes, properties and methods
   • Functions and constants
   • No need to add comments to tests
   • Use plain, easy to understand English