CH12: clarify comment about time between blocks

Comment explained why one 10-minute block doesn't give more security
than ten 1-minute blocks but didn't also warn about the problems with
1-minute blocks.

ch10.asciidoc

@@ -1181,7 +1181,10 @@ 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 three blocks, over a system with 10-minute blocks.
+wait for three blocks, over a system with 10-minute blocks.  The shorter
+the time between blocks, the more miner work is wasted on accidental
+forks (in addition to other problems), so many people prefer Bitcoin's
+10-minute blocks over shorter block intervals.
 ====
 
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