CH02::propagation: mention non-node relay and s/flooding/gossiping/

- Alice can send her transaction to software that will forward it to a
  node for her.  This is very common today.

- Previous text used the term "flooding" but the more common phrase for
  this particular propagation technique is "gossiping".

ch02.asciidoc

@@ -487,14 +487,16 @@ called "full verification nodes", or _full nodes_ for short.
 
 Alice's wallet application can send the new
 transaction to any Bitcoin node it is connected to over any type of
-connection: wired, WiFi, mobile, etc.  Her bitcoin wallet does not have
+connection: wired, WiFi, mobile, etc.  It can also send the transaction
+to another program (such as a block explorer) that will relay it to a
+node.  Her bitcoin wallet does not have
 to be connected to Bob's bitcoin wallet directly and she does not have
 to use the internet connection offered by the cafe, though both those
 options are possible, too. ((("propagation", "flooding
 technique")))((("flooding technique")))Any Bitcoin node that receives a
 valid transaction it has not seen before will immediately forward it to
 all other nodes to which it is connected, a propagation technique known
-as _flooding_. Thus, the transaction rapidly propagates out across the
+as _gossiping_. Thus, the transaction rapidly propagates out across the
 peer-to-peer network, reaching a large percentage of the nodes within a
 few seconds.