Update Doc for aws propagator default sampling behavior (open-telemetry#656)

Author: NathanielRN

sdk-extension/opentelemetry-sdk-extension-aws/README.rst

@@ -46,6 +46,11 @@ Next, use the provided `AwsXRayIdGenerator` to initialize the `TracerProvider`.
 Usage (AWS X-Ray Propagator)
 ----------------------------
 
+**NOTE**: Because the parent context extracted from the `X-Amzn-Trace-Id` header
+assumes the context is _not_ sampled by default, users should make sure to add
+`Sampled=1` to their `X-Amzn-Trace-Id` headers so that the child spans are
+sampled.
+
 Use the provided AWS X-Ray Propagator to inject the necessary context into
 traces sent to external systems.
 

sdk-extension/opentelemetry-sdk-extension-aws/src/opentelemetry/sdk/extension/aws/trace/propagation/aws_xray_format.py

@@ -18,9 +18,17 @@
 
 The **AWS X-Ray Propagator** provides a propagator that when used, adds a `trace
 header`_ to outgoing traces that is compatible with the AWS X-Ray backend service.
-This allows the trace context to be propagated when a trace span multiple AWS
+This allows the trace context to be propagated when a trace spans multiple AWS
 services.
 
+The same propagator setup is used to extract a context sent by external systems
+so that child span have the correct parent context.
+
+**NOTE**: Because the parent context parsed from the ``X-Amzn-Trace-Id`` header
+assumes the context is _not_ sampled by default, users should make sure to add
+``Sampled=1`` to their ``X-Amzn-Trace-Id`` headers so that the child spans are
+sampled.
+
 Usage
 -----