There are few post-commit tweaks in PR 11367:

* Removed the docstrings cache. I have not found significant difference, but this make the code simpler.
* Improved tests. Merged test_doc_writable and test_namedtuple_can_mutate_doc_of_descriptors_independently in the single test, added tests for immutability, hashing, field descriptors and help(), and rewriting old checks that used eval().
* Unified names of tp_descr_get and tp_descr_set functions. This will help to search implementations of custom descriptors.