Improve context propagation docstring (#209)

Author: dandavison

context_propagation/interceptor.py

@@ -47,7 +47,16 @@ def context_from_header(
 class ContextPropagationInterceptor(
     temporalio.client.Interceptor, temporalio.worker.Interceptor
 ):
-    """Interceptor that can serialize/deserialize contexts."""
+    """Interceptor that propagates a value through client, workflow and activity calls.
+
+    This interceptor implements methods `temporalio.client.Interceptor` and  `temporalio.worker.Interceptor` so that
+
+    (1) a user ID key is taken from context by the client code and sent in a header field with outbound requests
+    (2) workflows take this value from their task input, set it in context, and propagate it into the header field of
+        their outbound calls
+    (3) activities similarly take the value from their task input and set it in context so that it's available for their
+        outbound calls
+    """
 
     def __init__(
         self,