Update Azure doc fragment (#15450)

* Add default attribute to all option doc strings.

* Fix fragment file name.

lib/ansible/utils/module_docs_fragments/azure.py

@@ -31,31 +31,38 @@ options:
             - Active Directory username. Use when authenticating with an Active Directory user rather than service
               principal.
         required: false
+        default: null
     password:
         description:
             - Active Directory user password. Use when authenticating with an Active Directory user rather than service
               principal.
         required: false
+        default: null
     profile:
         description:
             - Security profile found in ~/.azure/credentials file.
         required: false
+        default: null
     subscription_id:
         description:
             - Your Azure subscription Id.
         required: false
+        default: null
     client_id:
         description:
             - Azure client ID. Use when authenticating with a Service Principal.
         required: false
+        default: null
     secret:
         description:
             - Azure client secret. Use when authenticating with a Service Principal.
         required: false
+        default: null
     tenant:
         description:
             - Azure tenant ID. Use when authenticating with a Service Principal.
         required: false
+        default: null
 
 requirements:
     - "python >= 2.7"