Document the case of activating a null value in the OT Shim. (#2225)
Document the case of activating a null value in the OT Shim. (#2225)

* Document the case of activating a null value in the OT Shim.

* Clarify the case of unsampled Spans.
open-telemetry/opentelemetry-specification,open-telemetry/opentelemetry-specification