Discriminator: Improve final example

This example was super confusing, which is mostly because it doesn't
indicate which schema the payload examples are being applied to. So, I
added the schema and an explanation of what is happening.

Author: jdesrosiers

versions/3.1.1.md

@@ -2772,6 +2772,11 @@ For example:
 ```yaml
 components:
   schemas:
+    MyResponseType:
+      oneOf:
+      - $ref: '#/components/schemas/Cat'
+      - $ref: '#/components/schemas/Dog'
+      - $ref: '#/components/schemas/Lizard'
     Pet:
       type: object
       required:
@@ -2809,7 +2814,12 @@ components:
             type: boolean
 ```
 
-a payload like this:
+The `MyResponseType` schema will use the discriminator defined by the `Pet`
+schema because it is part of the `Cat`, `Dog`, and `Lizard` schemas in the
+`oneOf`. The behavior if not all schemas define a `discriminator` and they are
+not all the same is undefined.
+
+Validated against the `MyResponseType` schema, a payload like this:
 
 ```json
 {