[Doc]: New-AzApiManagementSubscription

[jago933]

Description

In the New-AzApiManagementSubscription cmdlet reference doc , it is mentioned that the length of the primary key is up to 300 but it is actually up to 256 (same for the secondary key). I have checked the source code and also tested to set a value over 256 and it is failing.
URL : https://docs.microsoft.com/en-us/powershell/module/az.apimanagement/new-azapimanagementsubscription?view=azps-7.5.0

[dingmeng-xue]

@CaptainFanZzz , please check the implementation and update doc.