[VM/Service] Improve documentation of getInstancesAsList

This CL addresses some of the feedback in https://dart-review.googlesource.com/c/sdk/+/283380/comment/b86443cf_5d0f91bf/

TEST=CI

Issue: #51497
Change-Id: I26622287df8834bbacbe3fdfb95490350cecd4bb
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/284724
Reviewed-by: Ben Konyi <[email protected]>
Commit-Queue: Derek Xu <[email protected]>

Author: derekxu16

pkg/vm_service/lib/src/vm_service.dart

@@ -643,8 +643,9 @@ abstract class VmServiceInterface {
 
   /// The `getInstancesAsList` RPC is used to retrieve a set of instances which
   /// are of a specific class. This RPC returns an `InstanceRef` corresponding
-  /// to a Dart `List` that contains the requested instances. This is what
-  /// distinguishes this RPC from `getInstances`, which returns an
+  /// to a Dart `List<dynamic>` that contains the requested instances. This
+  /// `List` is not growable, but it is otherwise mutable. The response type is
+  /// what distinguishes this RPC from `getInstances`, which returns an
   /// `InstanceSet`.
   ///
   /// The order of the instances is undefined (i.e., not related to allocation

runtime/vm/service/service.md

@@ -882,9 +882,10 @@ See [InstanceSet](#instanceset).
 ```
 
 The _getInstancesAsList_ RPC is used to retrieve a set of instances which are of
-a specific class. This RPC returns an `@Instance` corresponding to a Dart `List`
-that contains the requested instances. This is what distinguishes this RPC from
-`getInstances`, which returns an `InstanceSet`.
+a specific class. This RPC returns an `@Instance` corresponding to a Dart
+`List<dynamic>` that contains the requested instances. This `List` is not
+growable, but it is otherwise mutable. The response type is what distinguishes
+this RPC from `getInstances`, which returns an `InstanceSet`.
 
 The order of the instances is undefined (i.e., not related to allocation order)
 and unstable (i.e., multiple invocations of this method against the same class