Document that item parameter of pytest_warning_captured hook is deprecated

changelog/4036.removal.rst

@@ -0,0 +1,9 @@
+The ``item`` parameter of ``pytest_warning_captured`` hook is now documented as deprecated. We realized only after
+the ``3.8`` release that this parameter is incompatible with ``pytest-xdist``.
+
+Our policy is to not deprecate features during bugfix releases, but in this case we believe it makes sense as we are
+only documenting it as deprecated, without issuing warnings which might potentially break test suites. This will get
+the word out that hook implementers should not use this parameter at all.
+
+In a future release ``item`` will always be ``None`` and will emit a proper warning when a hook implementation
+makes use of it.

src/_pytest/hookspec.py

@@ -562,6 +562,9 @@ def pytest_warning_captured(warning_message, when, item):
         * ``"runtest"``: during test execution.
 
     :param pytest.Item|None item:
+        **DEPRECATED**: This parameter is incompatible with ``pytest-xdist``, and will always receive ``None``
+        in a future release.
+
         The item being executed if ``when`` is ``"runtest"``, otherwise ``None``.
     """