MT#7747 API: Improve fee bulk upload documentation

lib/NGCP/Panel/Controller/API/BillingFees.pm

@@ -20,7 +20,7 @@ class_has 'api_description' => (
     is => 'ro',
     isa => 'Str',
     default => 
-        'Specifies the fees to be applied for a call if it matches the source or destination number of the call.'
+        'Specifies the fees to be applied for a call if it matches the source or destination number of the call. You can POST fees individually one-by-one using json. To bulk-upload fees, specify the Content-Type as "text/csv" and POST the CSV in the request body to the collection while specifying the the billing profile via URI parameters, like "/api/billingfees/?billing_profile_id=xx&purge_existing=true"'
 );
 
 class_has 'query_params' => (