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

Larry Hastings added the comment:

Py_INCREF usable as an rvalue sounds more like an accident than a deliberate feature

I'd go with "misfeature", but in no way is it accidental. The coding deliberately preserves the rvalue-ness of it, c.f. _Py_REF_DEBUG_COMMA. ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue17589> _______________________________________