Document assertrepr compare with assertion pass hook

[marko1olo]

Fixes #12062.

Documents that pytest_assertrepr_compare may be called for passing assertions when pytest_assertion_pass is implemented, and adds a regression test for that interaction.

Validation:

• SETUPTOOLS_SCM_PRETEND_VERSION_FOR_PYTEST=9.0.0 uv run --reinstall-package pytest --with-editable . python -m pytest testing/test_assertrewrite.py::TestAssertionPass::test_assertrepr_compare_called_for_pass_hook -q
• python -m compileall -q src\_pytest\hookspec.py testing\test_assertrewrite.py
• git diff --check HEAD~1 HEAD

Not run: full pytest test suite.