note on db_datawriter vs db_datareader

Author: AndreasWolter

docs/relational-databases/security/authentication-access/database-level-roles.md

@@ -71,7 +71,7 @@ For a list of all the permissions, see the [Database Engine Permissions](https:/
 |**db_accessadmin**|Members of the **db_accessadmin** fixed database role can add or remove access to the database for Windows logins, Windows groups, and [!INCLUDE[ssNoVersion](../../../includes/ssnoversion-md.md)] logins.|  
 |**db_backupoperator**|Members of the **db_backupoperator** fixed database role can back up the database.|  
 |**db_ddladmin**|Members of the **db_ddladmin** fixed database role can run any Data Definition Language (DDL) command in a database. Members of this role can potentially elevate their privileges by manipulating code that may get executed under high privileges and their actions should be monitored.|  
-|**db_datawriter**|Members of the **db_datawriter** fixed database role can add, delete, or change data in all user tables.|  
+|**db_datawriter**|Members of the **db_datawriter** fixed database role can add, delete, or change data in all user tables. In most use cases this role will be combined with **db_datareader** membership to allow reading the data that is to be modified.|  
 |**db_datareader**|Members of the **db_datareader** fixed database role can read all data from all user tables and views. User objects can exist in any schema except *sys* and *INFORMATION_SCHEMA*. |  
 |**db_denydatawriter**|Members of the **db_denydatawriter** fixed database role cannot add, modify, or delete any data in the user tables within a database.|  
 |**db_denydatareader**|Members of the **db_denydatareader** fixed database role cannot read any data from the user tables and views within a database.|