Trying to clarify the distinction between Public and the other fixed server roles

Trying to clarify the distinction between server roles with fixed permissions, and Public which has fixed membership and non-fixed permissions.

Author: davidbaxterbrowne

docs/relational-databases/system-catalog-views/sys-server-principals-transact-sql.md

@@ -44,7 +44,7 @@ monikerRange: ">=aps-pdw-2016||>=sql-server-2016||=sqlallproducts-allversions||>
 |**default_language_name**|**sysname**|Default language for this principal.|  
 |**credential_id**|**int**|ID of a credential associated with this principal. If no credential is associated with this principal, credential_id will be NULL.|  
 |**owning_principal_id**|**int**|The **principal_id** of the owner of a server role. NULL if the principal is not a server role.|  
-|**is_fixed_role**|**bit**|Returns 1 if the principal is one of the fixed server roles. For more information, see [Server-Level Roles](../../relational-databases/security/authentication-access/server-level-roles.md).|  
+|**is_fixed_role**|**bit**|Returns 1 if the principal is one of the built-in server roles with fixed permissions. For more information, see [Server-Level Roles](../../relational-databases/security/authentication-access/server-level-roles.md).|  
 
 ## Permissions  
  Any login can see their own login name, the system logins, and the fixed server roles. To see other logins, requires ALTER ANY LOGIN, or a permission on the login. To see user-defined server roles, requires ALTER ANY SERVER ROLE, or membership in the role.  
@@ -55,7 +55,7 @@ monikerRange: ">=aps-pdw-2016||>=sql-server-2016||=sqlallproducts-allversions||>
  The following query lists the permissions explicitly granted or denied to server principals.  
 
 > [!IMPORTANT]  
->  The permissions of fixed server roles do not appear in sys.server_permissions. Therefore, server principals may have additional permissions not listed here.  
+>  The permissions of fixed server roles (other than Public) do not appear in sys.server_permissions. Therefore, server principals may have additional permissions not listed here.  
   
 ```  
 SELECT pr.principal_id, pr.name, pr.type_desc,