Add docstring for new 'frame' parameter in pytest_warning_captured hook

Author: Philipp Loose

src/_pytest/hookspec.py

@@ -597,6 +597,10 @@ def pytest_warning_captured(warning_message, when, item, frame):
         in a future release.
 
         The item being executed if ``when`` is ``"runtest"``, otherwise ``None``.
+
+    :param types.FrameType frame:
+        The stackframe in which the captured warning occured. This is helpful to inspect the context
+        in which the warning was emitted.
     """