Improve documentation for trace.propagator.inject() (open-telemetry#3850)

Author: proffalken

opentelemetry-api/src/opentelemetry/propagate/__init__.py

@@ -109,8 +109,9 @@ def inject(
     """Uses the configured propagator to inject a Context into the carrier.
 
     Args:
-        carrier: An object that contains a representation of HTTP
-            headers. Should be paired with setter, which
+        carrier: the medium used by Propagators to read
+            values from and write values to.
+            Should be paired with setter, which
             should know how to set header values on the carrier.
         context: An optional Context to use. Defaults to current
             context if not set.