Update description for -SubscriptionId

I am an Azure book author, here is my propose update for this documentation.
-Subscription can be filled by subscription name or id , but while using subscription id , we should pass -SubscriptionId  as flag, tested with my past experience, and this also causes a misunderstanding from one of my readers.

Author: Melony QIN

src/ResourceManager/Profile/Commands.Profile/help/Set-AzureRmContext.md

@@ -164,7 +164,8 @@ Accept wildcard characters: False
 ```
 
 ### -Subscription
-Subscription Name or Id
+Subscription Name or Id 
+Whie using subscription Id as the parameter, this parameter is alias as -SubscriptionId 
 
 ```yaml
 Type: System.String