Docs: auto-close plugin now handles function-scoped fixtures

Describe the updated scoping rule: instances from test bodies and
function-scoped fixtures are closed automatically; session-, module-,
class- and package-scoped fixtures are exempt.

Refs #2692

docs/testing_plugins.rst

@@ -87,9 +87,18 @@ If you are using ``await datasette.client.get()`` and similar methods then you d
 Automatic cleanup of Datasette instances
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-Installing Datasette also installs a small pytest plugin that automatically calls :ref:`datasette_close` on any ``Datasette()`` instance constructed inside the body of a test function. This helps prevent large test suites from running out of file descriptors or leaking background threads from the hundreds of instances they may build up across a session.
+Installing Datasette also installs a small pytest plugin that automatically calls :ref:`datasette_close` on any ``Datasette()`` instance constructed during a test. This helps prevent large test suites from running out of file descriptors or leaking background threads from the hundreds of instances they may build up across a session.
 
-Instances created inside a pytest fixture are **not** closed by this plugin — pytest fixtures often create a single ``Datasette`` that is shared across many tests, and closing it automatically would break those tests. If you need a per-test instance and want to share it between multiple tests, create it inside a fixture rather than at the top level of a test function.
+The plugin closes:
+
+- Instances created in the body of a test function.
+- Instances created inside **function-scoped** pytest fixtures (the default scope — ``@pytest.fixture`` with no ``scope=`` argument, or ``scope="function"``).
+
+The plugin deliberately does **not** close:
+
+- Instances created inside higher-scoped fixtures (``scope="session"``, ``"module"``, ``"class"`` or ``"package"``). Those fixtures are typically designed to produce a single ``Datasette`` that is shared across many tests, and closing it automatically would break the tests that run after the first.
+
+In practice this means downstream projects rarely need to call ``ds.close()`` themselves — function-scoped fixtures and inline test code are both covered automatically, while long-lived shared fixtures keep working as before.
 
 If you need to opt out of this behavior, add the following to your ``pytest.ini`` (or equivalent):