Document missing type flags as part of Stable ABI

[da-woods]

The documentation is currently pretty inconsistent about listing the type flags as part of the Stable ABI. Some are listed and some aren't. I've hopefully got all the important ones.

I haven't done _Py_TPFLAGS_MATCH_SELF which is exposed in the Limited API but starts with an underscore.

I also haven't done things like Py_TPFLAGS_VALID_VERSION_TAG, Py_TPFLAGS_HAVE_FINALIZE, Py_TPFLAGS_HAVE_VERSION_TAG which are exposed in the Limited API but now don't do anything.