[issue21949] Document the Py_SIZE() macro.

[Roundup Robot]

Roundup Robot added the comment:

New changeset 63183596db79 by Gregory P. Smith in branch '2.7':
Per issue21949 and issue1629: Document the Py_SIZE, Py_TYPE, Py_REFCNT macros.
https://hg.python.org/cpython/rev/63183596db79

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nosy: +python-dev

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<http://bugs.python.org/issue21949>
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