Clarify that allow_broker is not applicable to ConfidentialClientApplication

It is applicable to PublicClientApplication and base class ClientApplication

Author: rayluo

msal/application.py

@@ -444,6 +444,8 @@ def __init__(
             New in version 1.19.0.
 
         :param boolean allow_broker:
+            This parameter is NOT applicable to :class:`ConfidentialClientApplication`.
+
             A broker is a component installed on your device.
             Broker implicitly gives your device an identity. By using a broker,
             your device becomes a factor that can satisfy MFA (Multi-factor authentication).