message: doc: Add note regarding ZMQ block notification
diff: @@ -111,7 +111,9 @@ using other means such as firewalling.
  Note that when the block chain tip changes, a reorganisation may occur
  and just the tip will be notified. It is up to the subscriber to
  -retrieve the chain from the last known block to the new tip.
  +retrieve the chain from the last known block to the new tip. Also note
  +that no notification occurs if the tip was in the active chain - this
  +is the case after calling invalidateblock RPC.
  There are several possibilities that ZMQ notification can get lost
  during transmission depending on the communication type you are
  