Enhance the comment ieee80211_add_channel() to avoid a

misunderstanding that the function does not work additive
when repeatedly called for diffferent bands.

Reviewed by:	avos (a few months ago)
MFC after:	2 weeks

sys/net80211/ieee80211.c

@@ -1386,6 +1386,8 @@ getflags(const uint8_t bands[], uint32_t flags[], int ht40, int vht80)
 
 /*
  * Add one 20 MHz channel into specified channel list.
+ * You MUST NOT mix bands when calling this.  It will not add 5ghz
+ * channels if you have any B/G/N band bit set.
  */
 /* XXX VHT */
 int