CH12: Add note about settlement being time-based not conf-based

ch10.asciidoc

@@ -1697,10 +1697,30 @@ Whereas a one-block fork might occur every day, a two-block fork occurs
 at most once every few weeks.
 
 Bitcoin's block interval of 10 minutes is a design compromise between
-fast confirmation times (settlement of transactions) and the probability
-of a fork. A faster block time would make transactions clear faster but
+fast confirmation times and the probability
+of a fork. A faster block time would make transactions seem to clear faster but
 lead to more frequent blockchain forks, whereas a slower block time
-would decrease the number of forks but make settlement slower.((("",
+would decrease the number of forks but make settlement seem slower.
+
+[NOTE]
+====
+Which is more secure: a transaction included in one block where the
+average time between blocks is 10 minutes, or a transaction included in
+a block with nine blocks built on top of it where the average time
+between blocks is one minute?  The answer is that they're equally
+secure.  A malicious miner wanting to double spend that transaction
+would need to do an amount of work equal to 10 minutes of the total
+network hash rate in order to create a chain with equal proof of work.
+
+Shorter times between blocks doesn't result in earlier settlement.  It's
+only advantage is providing weaker guarantees to people who are willing
+to accept those guarantees.  For example, if you're willing to accept
+three minutes of miners agreeing on the best block chain as sufficient
+security, you'd prefer a system with 1-minute blocks, where you could
+wait for there blocks, over a system with 10-minute blocks.
+====
+
+((("",
 startref="Bassemble10")))((("", startref="MACassembling10")))((("",
 startref="forks10")))((("", startref="BCTfork10")))