gh-135913: Document ob_refcnt, ob_type, ob_size

[encukou]

• Issue: Document ob_refcnt, ob_type, ob_size #135913

---

📚 Documentation preview 📚: https://cpython-previews--135914.org.readthedocs.build/

[colesbury]

I don't think this is accurate anymore:

cpython/Include/object.h

Lines 128 to 146 in a88b49c

	union {
	#if SIZEOF_VOID_P > 4
	PY_INT64_T ob_refcnt_full; /* This field is needed for efficient initialization with Clang on ARM */
	struct {
	# if PY_BIG_ENDIAN
	uint16_t ob_flags;
	uint16_t ob_overflow;
	uint32_t ob_refcnt;
	# else
	uint32_t ob_refcnt;
	uint16_t ob_overflow;
	uint16_t ob_flags;
	# endif
	};
	#else
	Py_ssize_t ob_refcnt;
	#endif
	_Py_ALIGNED_DEF(_PyObject_MIN_ALIGNMENT, char) _aligner;
	};

[encukou]

Hm, yes, the type isn't Py_ssize_t any more.
That's an issue in the current documentation too; guess it's time to reopen #125174 for a docs update.
Similarly, a lot of the docs talk about PyObject_HEAD_INIT setting the refcount to 1. I can't fix everything in this PR...