Clarify scoped fixture rerun support

[LouisDeconinck]

Fixes #183.

Clarify the currently supported fixture behavior in the README: setup failures and setup_class failures are rerun for class-, module-, package-, and session-scoped fixtures, while successful higher-scoped fixtures remain cached across a rerun and are torn down once afterward.

Add a package-scoped setup regression test and a towncrier documentation fragment.

Checks:

• PYTHONPATH=src python3 -m pytest -q (227 passed, 9 skipped)
• uvx --from pre-commit pre-commit run --files README.rst tests/test_pytest_rerunfailures.py changes/183.doc.rst
• git diff --check

[icemac]

Thanks for the PR. The new text is accurate (I verified the session-, module- and package-scope behaviour empirically), but a few things need adjusting before merge.

— Comment created by Claude

[LouisDeconinck]

Addressed the latest review round:

• README.rst: moved the "Fixture support" paragraph after the --reruns-delay/--reruns-delay-backoff-factor docs and gave it a --- heading, so it is a top-level section instead of a ~~~~~ subsection of "Re-run all failures".
• README.rst + docs/quickstart.rst: wrapped the section in .. START-FIXTURES / .. END-FIXTURES markers and included it from the quickstart page via start-after/end-before, matching the existing fragment includes. Verified with sphinx-build: it renders as its own section on the quickstart page.
• tests/test_pytest_rerunfailures.py: the package-scoped fixture now lives in pkg/conftest.py and is used by two test modules (pkg/test_foo.py, pkg/test_bar.py). The test asserts the fixture sets up twice in total (failed attempt + rerun) and is torn down once (["setup", "setup", "teardown"]); it fails under module scope, confirming it exercises package scope. Verified: 2 passed, 1 rerun.
• changes/183.doc.rst: unwrapped to a single line.

[icemac]

Thank you for this PR. 😃