Add internal documentation as JSDoc

[Reinachan]

When you're the sole developer, it's easy enough to remember what components exist, how they're used, and what they're used for. However, when developing in the open, it's a good idea to internally document these so we're certain everyone is on the same page.

Completion Criteria

Every component should have an associated JSDoc with at least a description and a list of all parameters. Where it makes sense, it should also include a usage example.

• useGetUser
• useGetUsers