Document permissions required for Remove-AzADUser

[jeking3]

Description of the new feature

Please document the permission requirements of Remove-AzADUser. I have a service principal configured with the owner role and the application privileges that allow me to call Add-AzADUser successfully. When I try calling Remove-AzADUser it fails with a permissions error.

Related to #6843.

Proposed implementation details (optional)

[dingmeng-xue]

@dcaro and @mikefrobbins , how can we get the permission information?