feat: add extensions parameter documentation in ClientFactory and update header function docstring

Author: sokoliva

src/a2a/client/client_factory.py

@@ -145,6 +145,7 @@ async def connect(  # noqa: PLR0913
             A2AAgentCardResolver.get_agent_card as the http_kwargs parameter.
           extra_transports: Additional transport protocols to enable when
             constructing the client.
+          extensions: List of extensions to be activated.
 
         Returns:
           A `Client` object.
@@ -190,6 +191,7 @@ def create(
           interceptors: A list of interceptors to use for each request. These
             are used for things like attaching credentials or http headers
             to all outbound requests.
+          extensions: List of extensions to be activated.
 
         Returns:
           A `Client` object.

src/a2a/extensions/common.py

@@ -34,6 +34,7 @@ def update_extension_header(
     active_extensions: list[str] | None,
     new_extensions: list[str] | None,
 ) -> tuple[dict[str, Any], list[str] | None]:
+    """Update the X-A2A-Extensions header and update active extensions."""
     if new_extensions:
         active_extensions = new_extensions
     if active_extensions: