Updated UserPrivilege errors with InvalidArgument

Adding InvalidArgument error to the UserPrivilege property, which will
be returned, when specified user privilege is not in the list as per
the xyz.openbmc_project.User.Manager - AllPrivileges property

Tested:
1. Update the UserPrivilege to unknown privilege.
Command: busctl set-property xyz.openbmc_project.User.Manager
         /xyz/openbmc_project/user/user3 xyz.openbmc_project.User.
         Attributes UserPrivilege s "test"

Response: Failed to set property UserPrivilege on interface xyz.openbmc_
         project.User.Attributes: Invalid argument was given.

2. Update the UserPrivilege to "priv-user"
Command: busctl set-property xyz.openbmc_project.User.Manager
         /xyz/openbmc_project/user/user3 xyz.openbmc_project.User.
         Attributes UserPrivilege s "priv-user"

Response:

Signed-off-by: Chalapathi Venkataramashetty <chalapathix.venkataramashetty@intel.com>
Change-Id: I82ecc7873ec23e99291f80c4f00481d556c25931
diff --git a/xyz/openbmc_project/User/Attributes.interface.yaml b/xyz/openbmc_project/User/Attributes.interface.yaml
index 24df81b..50d1816 100644
--- a/xyz/openbmc_project/User/Attributes.interface.yaml
+++ b/xyz/openbmc_project/User/Attributes.interface.yaml
@@ -14,9 +14,11 @@
     - name: UserPrivilege
       type: string
       description: >
-          Privilege of the user.
+          Privilege of the user. Acceptable values of the system are listed
+          under AllPrivileges property under User.Manager interface.
       errors:
           - xyz.openbmc_project.Common.Error.InternalFailure
+          - xyz.openbmc_project.Common.Error.InvalidArgument
 
     - name: UserEnabled
       type: boolean