bpo-43908: Document Static Types in the C API (GH-25710)

Update also PyTypeObject structure definition in the doc.
diff --git a/Include/cpython/object.h b/Include/cpython/object.h
index 58e4d2b..84c60e5 100644
--- a/Include/cpython/object.h
+++ b/Include/cpython/object.h
@@ -186,6 +186,8 @@ typedef struct {
  * backwards-compatibility */
 typedef Py_ssize_t printfunc;
 
+// If this structure is modified, Doc/includes/typestruct.h should be updated
+// as well.
 struct _typeobject {
     PyObject_VAR_HEAD
     const char *tp_name; /* For printing, in format "<module>.<name>" */