Update Set-AzureRmVMADDomainExtension.md

A little context on options is helpful.

Author: DanManson

src/ResourceManager/Compute/Commands.Compute/help/Set-AzureRmVMADDomainExtension.md

@@ -121,6 +121,7 @@ Default value: None
 Accept pipeline input: True (ByPropertyName)
 Accept wildcard characters: False
 ```
+For join options see [fJoinOptions](https://docs.microsoft.com/en-us/windows/desktop/api/lmjoin/nf-lmjoin-netjoindomain)
 
 ### -Location
 Specifies the location of the virtual machine.