R1:630fb06c428e

Document how to use `harbormaster.sendmessage` to report lint and unit results

Summary: Fixes T7419. This doesn't really do anything, just adds documentation.

Test Plan:
- Read the documentation:

{F688899}

- Created a build plan which makes an HTTP request to `example.com` and waits for a result.
- Ran that build plan manually.
- Called `harbormaster.sendmessage` manually with the example lint/unit values to provide a result.
- Saw the results report correctly and the message ("fail") process…