Document current usage recommendations

[garvankeeley]

1. is not thread-safe, call only from main
2. is only for CorePing and UI events which are inherently rate-limited by user interaction.

[Sdaswani]

Per 1, can we also clarify that the call is non-blocking, so calling from main is relatively safe?

[garvankeeley]

Will do, you are correct, all calls are main-thread async. They can result in main thread contention if called at excessive rate.

[Sdaswani]

Yeah so we need to change that down the line. It's not good API behavior to expect clients to build their own rate limiting.

[garvankeeley]

See also https://github.com/mozilla-mobile/telemetry-ios/issues/12