bonding: update bonding.txt for primary description

Signed-off-by: Ding Tianhong <dingtianhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/Documentation/networking/bonding.txt b/Documentation/networking/bonding.txt
index a4d925e..5cdb229 100644
--- a/Documentation/networking/bonding.txt
+++ b/Documentation/networking/bonding.txt
@@ -657,7 +657,8 @@
 	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