Update help for ARM cmdlets

[vivsriaus]

No description provided.

[hovsepm]

@vivsriaus there 3 more cmdlets without help. could you add them as well? here they are:

1. Get-AzureRmADApplication implemented by class Microsoft.Azure.Commands.ActiveDirectory.GetAzureADApplicationCommand
2. Remove-AzureRmResourceGroupDeployment implemented by class Microsoft.Azure.Commands.Resources.ResourceGroups.RemoveAzureResourceGroupDeploymentCommand
3. Test-AzureRmResourceGroupDeployment implemented by class Microsoft.Azure.Commands.Resources.ResourceGroupDeployments.TestAzureResourceGroupDeploymentCommand

[vivsriaus]

@hovsepm #2 and #3 are taken care of. For #1, I'll let @shuagarw who implemented the cmdlet, add the help content. Thanks

[markcowl]

on demand run here: http://azuresdkci.cloudapp.net/view/1-AzurePowerShell/job/powershell-demand/552/