comment: Add note about typing question and reference to issue.

Possibly fix-able in AS3.

Ref: https://github.com/apollographql/apollo-server/pull/3988/files#r421632153
Ref: #4103

packages/apollo-server-plugin-base/src/index.ts

@@ -45,6 +45,14 @@ export {
   GraphQLRequestContextWillSendResponse,
 };
 
+// Typings Note! (Fix in AS3?)
+//
+// There are a number of types in this module which are specifying `void` as
+// their return type, despite the fact that we _are_ observing the value.
+// It's possible those should instead be `undefined`.  For more details, see
+// the issue that was logged as a result of this discovery during (unrelated) PR
+// review: https://github.com/apollographql/apollo-server/issues/4103
+
 export interface ApolloServerPlugin<
   TContext extends BaseContext = BaseContext
 > {