[fs] Document and expose the AsyncFS interface

[danking]

What happened?

• [ ] Document the AsyncFS interface, RouterAsyncFS, and the three clouds.
• [ ] Do a critical pass of the interfaces and make sure public methods don't have underscores and private methods do. When in doubt, make it private.
• [ ] Same as above but for parameters. For this, be particularly aggressive about making parameters private!
• [ ] Downgrade positional arguments to keyword-arguments as much as is feasible without severely degrading UX.

Version

0.2.126

Relevant log output

No response