Mention ec_multi*

doc/TODO.smpnet

@@ -1,4 +1,4 @@
-$NetBSD: TODO.smpnet,v 1.15 2017/10/12 03:31:39 ozaki-r Exp $
+$NetBSD: TODO.smpnet,v 1.16 2017/10/25 07:35:40 ozaki-r Exp $
 
 MP-safe components
 ==================
@@ -163,3 +163,12 @@ Scalability
    is O(n)
  - opencrypto(9)'s crypto_newsession()/crypto_freesession() aren't scalable
    as they are serialized by one mutex
+
+ec_multi* of ethercom
+---------------------
+
+ec_multiaddrs and ec_multicnt of struct ethercom and items listed in
+ec_multiaddrs must be protected by ec_lock.  The core of ethernet subsystem is
+already MP-safe, however, device drivers that use the data should also be fixed.
+A typical change should be to protect manipulations of the data via ETHER_*
+macros such as ETHER_FIRST_MULTI by ETHER_LOCK and ETHER_UNLOCK.