Windows.System.User.GetFromId: add Remarks (null return, NonRoamableId note) - fixes #1195

Author: GrantMeStrength

windows.system/user_getfromid_2093276151.md

@@ -20,6 +20,9 @@ The Id of the user to get.
 The user with the given Id.
 
 ## -remarks
+If no user with the given *nonRoamableId* is found, this method returns **null**. Always check the return value before using it.
+
+A *nonRoamableId* can be obtained from the [NonRoamableId](user_nonroamableid.md) property of a [User](user.md) object. This identifier is device-specific and is not roamed to other devices.
 
 ## -examples