fix(client): clarify the purpose of the extensions parameter in Client constructor

Author: sokoliva

src/a2a/client/client.py

@@ -100,6 +100,7 @@ def __init__(
         Args:
             consumers: A list of callables to process events from the agent.
             middleware: A list of interceptors to process requests and responses.
+            extensions: A list of extension URIs the client supports.
         """
         if middleware is None:
             middleware = []