We need expanded documentation for TypedCommand. In general, it lacks documentation in these ways:

• We should introduce a canonical example TypedCommand implementation. We will probably need to introduce a toy jstest to use the command. This mirrors the documentation examples for ServerParameters here.
• We should create a document describing in prose how and why one creates a TypedCommand as docs/typed_command.md which is referenced in src/mongo/db/README.md.
• We should expand the class comment for TypedCommand here to detail how to extend its nested classes and why the hierarchy is as it is---for dependent type lookup on TypedCommand<T>::Invocation. Implementers seem to have the most frustration when attempting to understanding this web of types.