Clarify non-blocking requirement from span API End (open-telemetry#1555)

CHANGELOG.md

@@ -18,6 +18,7 @@ release.
 - Clarify the description for the `TraceIdRatioBased` sampler needs to include the sampler's sampling ratio. ([#1536](https://github.com/open-telemetry/opentelemetry-specification/pull/1536))
 - Define the fallback tracer name for invalid values.
   ([#1534](https://github.com/open-telemetry/opentelemetry-specification/pull/1534))
+- Clarify non-blocking requirement from span API End. ([#1555](https://github.com/open-telemetry/opentelemetry-specification/pull/1555))
 - Remove the Included Propagators section from trace API specification that was a duplicate of the Propagators Distribution of the context specification. ([#1556](https://github.com/open-telemetry/opentelemetry-specification/pull/1556))
 - Remove the Baggage API propagator notes that conflict with the API Propagators Operations section and fix [#1526](https://github.com/open-telemetry/opentelemetry-specification/issues/1526). ([#1575](https://github.com/open-telemetry/opentelemetry-specification/pull/1575))
 

specification/trace/api.md

@@ -617,7 +617,14 @@ Parameters:
 - (Optional) Timestamp to explicitly set the end timestamp.
   If omitted, this MUST be treated equivalent to passing the current time.
 
-This API MUST be non-blocking.
+Expect this operation to be called in the "hot path" of production
+applications. It needs to be designed to complete fast, if not immediately.
+This operation itself MUST NOT perform blocking I/O on the calling thread.
+Any locking used needs be minimized and SHOULD be removed entirely if
+possible. Some downstream SpanProcessors and subsequent SpanExporters called
+from this operation may be used for testing, proof-of-concept ideas, or
+debugging and may not be designed for production use themselves. They are not
+in the scope of this requirement and recommendation.
 
 #### Record Exception