Publish UnknownMarkWarning as part of the public API and docs

doc/en/warnings.rst

@@ -420,3 +420,5 @@ The following warning types ares used by pytest and are part of the public API:
 .. autoclass:: pytest.RemovedInPytest4Warning
 
 .. autoclass:: pytest.PytestExperimentalApiWarning
+
+.. autoclass:: pytest.PytestUnknownMarkWarning

src/_pytest/mark/structures.py

@@ -12,7 +12,7 @@ from ..compat import MappingMixin
 from ..compat import NOTSET
 from _pytest.deprecated import PYTEST_PARAM_UNKNOWN_KWARGS
 from _pytest.outcomes import fail
-from _pytest.warning_types import UnknownMarkWarning
+from _pytest.warning_types import PytestUnknownMarkWarning
 
 EMPTY_PARAMETERSET_OPTION = "empty_parameter_set_mark"
 
@@ -318,7 +318,7 @@ class MarkGenerator(object):
                         "Unknown pytest.mark.%s - is this a typo?  You can register "
                         "custom marks to avoid this warning - for details, see "
                         "https://docs.pytest.org/en/latest/mark.html" % name,
-                        UnknownMarkWarning,
+                        PytestUnknownMarkWarning,
                     )
 
         return MarkDecorator(Mark(name, (), {}))

src/_pytest/warning_types.py

@@ -9,7 +9,7 @@ class PytestWarning(UserWarning):
     """
 
 
-class UnknownMarkWarning(PytestWarning):
+class PytestUnknownMarkWarning(PytestWarning):
     """
     Bases: :class:`PytestWarning`.
 

src/pytest.py

@@ -37,6 +37,7 @@ from _pytest.recwarn import deprecated_call
 from _pytest.recwarn import warns
 from _pytest.warning_types import PytestDeprecationWarning
 from _pytest.warning_types import PytestExperimentalApiWarning
+from _pytest.warning_types import PytestUnknownMarkWarning
 from _pytest.warning_types import PytestWarning
 from _pytest.warning_types import RemovedInPytest4Warning
 
@@ -72,6 +73,7 @@ __all__ = [
     "raises",
     "register_assert_rewrite",
     "RemovedInPytest4Warning",
+    "PytestUnknownMarkWarning",
     "Session",
     "set_trace",
     "skip",

tox.ini

@@ -169,7 +169,7 @@ filterwarnings =
     # Do not cause SyntaxError for invalid escape sequences in py37.
     default:invalid escape sequence:DeprecationWarning
     # ignore use of unregistered marks, because we use many to test the implementation
-    ignore::_pytest.warning_types.UnknownMarkWarning
+    ignore::_pytest.warning_types.PytestUnknownMarkWarning
 pytester_example_dir = testing/example_scripts
 markers =
     issue