Apply Peter's suggestion

Co-authored-by: Peter Bierma <zintensitydev@gmail.com>

Author: StanFromIreland

Doc/c-api/type.rst

@@ -138,7 +138,11 @@ Type Objects
    Return non-zero if the type object *type* sets the subclass flag *flag*.
    Subclass flags are denoted by
    :c:macro:`Py_TPFLAGS_*_SUBCLASS <Py_TPFLAGS_LONG_SUBCLASS>`.
-   This function is used by most ``_Check`` functions.
+   This function is used by many ``_Check`` functions for common types.
+
+   .. seealso::
+       :c:func:`PyObject_TypeCheck`, which is used as a slower alternative in
+       ``_Check`` functions for types that don't come with subclass flags.
 
 
 .. c:function:: int PyType_IS_GC(PyTypeObject *o)