Improve the Recipient description for com_messages Ref: #4738

Ref: https://github.com/joomla/joomla-cms/issues/4738

@brianteeman Can you have a look into the english?
+COM_MESSAGES_FIELD_USER_ID_TO_DESC="You must select a recipient. Note that you can only select users with specific access as recipients. A user needs to have one of the following access levels: core.admin (global), core.manage (in com_messages) or core.login.admin. All other users, specifically the Front End users without backend access, cannot access the messages component so you cannot select them as recipients."