Docs: Note lifetime of temporary directories

Explanation: The default handling of these lifetimes is done in
`tmpdir.TempPathFactory.getbasetemp`, which passes `keep=3` to
`pathlib.make_numbered_dir_with_cleanup`.

GH Issue: pytest-dev#8036

src/_pytest/tmpdir.py

@@ -174,6 +174,11 @@ def tmpdir(tmp_path: Path) -> py.path.local:
     function invocation, created as a sub directory of the base temporary
     directory.
 
+    By default, a new base temporary directory is created each test session,
+    and old bases are removed after 3 sessions, to aid in debugging. If
+    ``--basetemp`` is used then it is cleared each session. See :ref:`base
+    temporary directory`.
+
     The returned object is a `py.path.local`_ path object.
 
     .. _`py.path.local`: https://py.readthedocs.io/en/latest/path.html
@@ -187,6 +192,11 @@ def tmp_path(request: FixtureRequest, tmp_path_factory: TempPathFactory) -> Path
     function invocation, created as a sub directory of the base temporary
     directory.
 
+    By default, a new base temporary directory is created each test session,
+    and old bases are removed after 3 sessions, to aid in debugging. If
+    ``--basetemp`` is used then it is cleared each session. See :ref:`base
+    temporary directory`.
+
     The returned object is a :class:`pathlib.Path` object.
     """