Docs: Note lifetime of temporary directories
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: #8036

nicoddemus/pytest,nicoddemus/pytest,RonnyPfannschmidt/pytest,pytest-dev/pytest,Akasurde/pytest,markshao/pytest