CH02::mining: clarify that putting txes in blocks isn't hard

It's just forming the header correctly that's hard.

Also remove the word trust, since Bitcoin doesn't depend on "trust in
computation".

ch02.asciidoc

@@ -521,9 +521,11 @@ network. It does not become part of the _blockchain_ until it is
 verified and included in a block by a process called _mining_. See
 <<mining>> for a detailed explanation.
 
-The Bitcoin system of trust is based on computation. Transactions are
-bundled into _blocks_, which require an enormous amount of computation
-to prove, but only a small amount of computation to verify as proven.
+The Bitcoin system of counterfeit protection is based on computation.
+Transactions are bundled into _blocks_.  Blocks have a very small header
+that must be formed in a very specific way, requiring an enormous
+amount of computation to get right--but only a small amount of
+computation to verify as correct.
 The mining process serves two purposes in bitcoin:
 
 * ((("mining and consensus", "consensus rules", "security provided