Per issue21949 and issue1629: Document the Py_SIZE, Py_TYPE, Py_REFCNT macros.

This was already done in the 3.5 branch; this just does the same in the 2.7
branch.

Doc/c-api/structures.rst

@@ -69,6 +69,37 @@ These macros are used in the definition of :c:type:`PyObject` and
    expansion varies depending on the definition of :c:macro:`Py_TRACE_REFS`.
 
 
+.. c:macro:: Py_TYPE(o)
+
+   This macro is used to access the :attr:`ob_type` member of a Python object.
+   It expands to::
+
+      (((PyObject*)(o))->ob_type)
+
+   .. versionadded:: 2.6
+
+
+.. c:macro:: Py_REFCNT(o)
+
+   This macro is used to access the :attr:`ob_refcnt` member of a Python
+   object.
+   It expands to::
+
+      (((PyObject*)(o))->ob_refcnt)
+
+   .. versionadded:: 2.6
+
+
+.. c:macro:: Py_SIZE(o)
+
+   This macro is used to access the :attr:`ob_size` member of a Python object.
+   It expands to::
+
+      (((PyVarObject*)(o))->ob_size)
+
+   .. versionadded:: 2.6
+
+
 .. c:macro:: PyObject_HEAD_INIT(type)
 
    This is a macro which expands to initialization values for a new