[PR #13685/12bde8af backport][8.4.x] Docs+Tests: clarify special discovery of pytest_generate_tests

[patchback]

This is a backport of PR #13685 as merged into main (12bde8a).

Summary
This PR improves the documentation and tests around the special discovery rules of pytest_generate_tests. Unlike most hooks (which must live in conftest.py or plugins), pytest_generate_tests is also discovered when defined directly inside test modules or classes. This PR clarifies that behavior in the docs and adds a minimal self-test that shows the difference.

Motivation
New users often get confused about where hooks should be placed. By explicitly pointing out that pytest_generate_tests is the only exception to the “hooks go in conftest/plugins” rule, we reduce misunderstandings and make the docs easier to follow.

Changes

• Updated doc/en/how-to/writing_hook_functions.rst with a clear note on the exception for pytest_generate_tests.
• Updated doc/en/how-to/parametrize.rst with a short reminder in the relevant section.
• Added testing/test_pytest_generate_tests_discovery.py, which demonstrates that pytest_generate_tests inside a test module works, while pytest_terminal_summary does not.

Tests

• Confirms pytest_generate_tests in a test module parametrizes correctly.
• Confirms another hook (pytest_terminal_summary) inside a test module is ignored.

Docs
Only minimal “Note:” additions with cross-links, written for clarity and easy review.

Notes
No behavior changes — purely docs and tests. Pre-commit, mypy, and docs build all pass locally.

Related Issue
Closes #13577