More fixes for observable array type

* Observable array type must not be nullable,
* Disallow observable array type being the inner type of an observable array type.

Author: EdgarChen

index.bs

@@ -6368,7 +6368,8 @@ character after an existing type.
 The [=nullable types/inner type=] must not be:
 
 *   {{any}},
-*   a [=Promise type=],
+*   a [=promise type=],
+*   an [=observable array type=],
 *   another nullable type, or
 *   a [=union type=] that itself [=includes a nullable type=]
     or has a dictionary type as one of its [=flattened member types=].
@@ -6825,7 +6826,8 @@ An <dfn id="dfn-observable-array-type" export>observable array type</dfn> is a p
 whose values are references to a combination of a mutable list of objects of type |T|, as well as
 behavior to perform when author code modifies the contents of the list.
 
-The parameterized type must not be a [=dictionary type=], [=sequence type=], or [=record type=].
+The parameterized type |T| must not be a [=dictionary type=], [=sequence type=], [=record type=],
+or [=observable array type=]. However, |T| may be nullable.
 
 Similar to [=sequence types=] and [=frozen array types=], observable array types wrap around
 ECMAScript array types, imposing additional semantics on their usage.