chore: Clean-up Documentation for Mapped SnakeCased Properties

Today, we manually document mapped snakeCase'd keys in a number of places to improve ergonomics for customers manually instantiating AuthClients as of googleapis/google-auth-library-nodejs#1663.

At the time of writing, [keys remapped via `as`](https://www.typescriptlang.org/docs/handbook/2/mapped-types.html#key-remapping-via-as) do not pass the [original keys' jsdoc](https://github.com/microsoft/TypeScript/issues/50715), requiring us to manually write the documentation for each key. Once this has been resolved on the TypeScript side we should clean up the documentation and types throughout the library.

Implementation Notes:
- Most places where this is a problem should reference this issue's id; it is best to start there.

Dependencies:
- https://github.com/googleapis/google-auth-library-nodejs/pull/1663
- https://github.com/microsoft/TypeScript/issues/50715