Docs+Tests: clarify special discovery of pytest_generate_tests

- Document that pytest_generate_tests is also discovered in test modules/classes.
- Clarify other hooks must live in conftest.py or plugins; add cross-links.
- Add a tiny pytester test demonstrating the behavior.
- Pure docs/tests; no behavior change.

Author: CortexShadow

doc/en/how-to/parametrize.rst

@@ -240,6 +240,13 @@ command line option and the parametrization of our test function:
         if "stringinput" in metafunc.fixturenames:
             metafunc.parametrize("stringinput", metafunc.config.getoption("stringinput"))
 
+.. note::
+
+    The :hook:`pytest_generate_tests` hook can also be implemented directly in a test
+    module or inside a test class; unlike other hooks, pytest will discover it there
+    as well. Other hooks must live in a :ref:`conftest.py <localplugin>` or a plugin.
+    See :ref:`writinghooks`.
+
 If we now pass two stringinput values, our test will run twice:
 
 .. code-block:: pytest

doc/en/how-to/writing_hook_functions.rst

@@ -235,6 +235,12 @@ Example:
         """
         print(config.hook)
 
+.. note::
+
+    Unlike other hooks, the :hook:`pytest_generate_tests` hook is also discovered when
+    defined inside a test module or test class. Other hooks must live in
+    :ref:`conftest.py plugins <localplugin>` or external plugins.
+    See :ref:`parametrize-basics` and the :ref:`hook-reference`.
 
 .. _`addoptionhooks`:
 

testing/test_pytest_generate_tests_discovery.py

@@ -0,0 +1,16 @@
+def test_generate_tests_discovery_in_test_module_and_not_other_hooks(pytester):
+    pytester.makepyfile(
+        """
+        def pytest_generate_tests(metafunc):
+            if "x" in metafunc.fixturenames:
+                metafunc.parametrize("x", [1, 2])
+
+        def pytest_terminal_summary(terminalreporter):
+            raise AssertionError("should not be called")
+
+        def test_x(x):
+            assert x in (1, 2)
+        """
+    )
+    result = pytester.runpytest()
+    result.assert_outcomes(passed=2)