[issue17589] Make documentation about macros in C API explicit about rvalue vs statement

[Amaury Forgeot d'Arc]

Amaury Forgeot d'Arc added the comment:

There are some extension modules (pytables) that do
  return Py_INREF(x), x;
and Py_RETURN_NONE is also defined with a comma expression.

Oh, and Cython:
#define __Pyx_PyBool_FromLong(b) ((b) ? (Py_INCREF(Py_True), Py_True) : (Py_INCREF(Py_False), Py_False))

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nosy: +amaury.forgeotdarc

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<http://bugs.python.org/issue17589>
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