Add an example query to the DisableIntrospection docs (#3898)

Author: DoctorJohn

docs/extensions/disable-introspection.md

@@ -35,3 +35,37 @@ schema = strawberry.Schema(
 ## API reference:
 
 _No arguments_
+
+## Example query:
+
+Running any query including the introspection field `__schema` will result in an
+error. Consider the following query, for example:
+
+```graphql
+query {
+  __schema {
+    __typename
+  }
+}
+```
+
+Running it against the schema with the `DisableIntrospection` extension enabled
+will result in an error response indicating that introspection has been
+disabled:
+
+```json
+{
+  "data": null,
+  "errors": [
+    {
+      "message": "GraphQL introspection has been disabled, but the requested query contained the field '__schema'.",
+      "locations": [
+        {
+          "line": 2,
+          "column": 3
+        }
+      ]
+    }
+  ]
+}
+```