Add note about extensions not using ext ID as entry key

index.bs

@@ -5235,13 +5235,21 @@ A [=[RP]=] simultaneously requests the use of an extension and sets its [=client
 {{PublicKeyCredentialCreationOptions/extensions}} option to the {{CredentialsContainer/create()}} or {{CredentialsContainer/get()}} call.
 The entry key is the [=extension identifier=] and the value is the [=client extension input=].
 
+Note: Other documents have specified extensions where the extension input does not always use the [=extension identifier=]
+as the entry key.
+New extensions SHOULD follow the above convention.
+
 <pre class="example" highlight="js">
     var assertionPromise = navigator.credentials.get({
         publicKey: {
-            // The challenge is produced by the server; see the Security Considerations
-            challenge: new Uint8Array([4,99,22 /* 29 more random bytes generated by the server */]),
+            // Other members omitted for brevity
             extensions: {
-                "webauthnExample_foobar": 42
+                // An extension with identifier "webauthnExample_foobar"
+                // and 2 input parameters
+                "webauthnExample_foobar": {
+                  foo: 42,
+                  bar: "barfoo"
+                }
             }
         }
     });