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As far as I can tell, the only mentions of the C API version macros are:
1. In the prose for the stable ABI section (incidentally: this section has a typo in the title)
2. In the documentation for sys.hexversion
There should be a clear "API and ABI Versioning" section in the C API docs that covers:
PY_LIMITED_API
PY_HEX_VERSION
PY_MAJOR_VERSION
PY_MINOR_VERSION
PY_MICRO_VERSION
PY_RELEASE_LEVEL
PY_RELEASE_SERIAL

Attaching patch with the initial documentation of the macros. It also fixes the typo in the title of stable API section.

Adding the updated patch with changes as suggested in the review

version 3 of the patch where sys.hexversion table is moved and now a link to api and abi versioning given.

version 4 of the patch contains a new column in table as Bytes as suggested by Ezio Melotti

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).

Attached version 5 of the patch with changes based on suggestions of ncoghlan

New changeset adeafab9a18f by Nick Coghlan in branch '3.3':
Close #15465: Document C API version macros
http://hg.python.org/cpython/rev/adeafab9a18f
New changeset a1373861f62f by Nick Coghlan in branch 'default':
Merge fix for #15465 from 3.3
http://hg.python.org/cpython/rev/a1373861f62f