bonding: update bonding.txt for primary description

Signed-off-by: NDing Tianhong <dingtianhong@huawei.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/networking/bonding.txt

@@ -657,7 +657,8 @@ primary
 	one slave is preferred over another, e.g., when one slave has
 	higher throughput than another.
 
-	The primary option is only valid for active-backup mode.
+	The primary option is only valid for active-backup(1),
+	balance-tlb (5) and balance-alb (6) mode.
 
 primary_reselect