Mostly looks good to me, but the second sentence in the first paragraph of the new section should be deleted. The relationship between PY_LIMITED_API and PY_VERSION_HEX is already covered in the stable ABI docs:

"""In some cases, the stable ABI needs to be extended with new functions. Extensions modules wishing to use these new APIs need to set Py_LIMITED_API to the PY_VERSION_HEX value of the minimum Python version they want to support (e.g. 0x03030000 for Python 3.3). Such modules will work on all subsequent Python releases, but fail to load (because of missing symbols) on the older releases."""

(As a separate issue, it may be worth doing an editing pass on the stable ABI docs - there are a few typos in there, and the layout is a bit odd for a reference doc. We shouldn't hold up this patch for that, though).