Clarification around user_sid column

I'm clarifying that the user_sid information in this table may not be up-to-date all the time:
- The information in the table is up-to-date when the proxy is added;
- However, external actions (typically an ALTER CREDENTIAL ...) may cause the information to become stale
- A way to make sure the information is current is to execute sp_update_proxy

Author: Matteo-T

docs/relational-databases/system-tables/dbo-sysproxies-transact-sql.md

@@ -33,7 +33,7 @@ ms.author: chadam
 |**credential_id**|**int**|ID of the credential that the proxy account uses.|  
 |**enabled**|**tinyint**|Status of the proxy account:<br /><br /> **0** = Disabled. **1** = Enabled.|  
 |**description**|**nvarchar(512)**|Description that the user entered when the proxy account was created.|  
-|**user_sid**|**varbinary(85)**|Microsoft Windows *security_identifier* of the user or group associated with the proxy credential.|  
+|**user_sid**|**varbinary(85)**|Microsoft Windows *security_identifier* of the user or group associated with the proxy credential, at the time the proxy is added. To make sure the information is up to date (e.g. after an `ALTER CREDENTIAL` was issued), please run sp_update_proxy to trigger a refresh of the information.|
 |**credential_date_created**|**datetime**|Date and time that the credential was created.|  
 
 ## Remarks