CH01::Mining: Clarify that miners add security to transactions

Previous text said they "verify" transactions, but that's not always the
case (e.g. validationless mining) and it may give readers the impression
that the entities primarily responsible for verifying transactions are
miners---when it's actually users who are ultimately responsible for
verifying the transactions they care about.

ch01.asciidoc

@@ -31,15 +31,16 @@ prerequisite to spending bitcoin, putting the control entirely in the
 hands of each user.
 
 Bitcoin is a distributed, peer-to-peer system. As such, there is no
-"central" server or point of control. Bitcoins, i.e. units of bitcoin,
+"central" server or point of control. Units of bitcoin
 are created through a process called "mining," which involves competing
-to find solutions to a mathematical problem while processing Bitcoin
+to find solutions to a mathematical problem that references a list of recent Bitcoin
 transactions. Any participant in the Bitcoin network (i.e., anyone using
 a device running the full Bitcoin protocol stack) may operate as a
-miner, using their computer's processing power to verify and record
-transactions. Every 10 minutes, on average, a Bitcoin miner can validate
-the transactions of the past 10 minutes and is rewarded with brand new
-bitcoin. Essentially, Bitcoin mining decentralizes the currency-issuance
+miner, using their computer's processing power to help secure
+transactions. Every 10 minutes, on average, a Bitcoin miner can add security to
+past transactions and is rewarded with both brand new
+bitcoin and the fees paid by recent transactions. Essentially, Bitcoin
+mining decentralizes the currency-issuance
 and clearing functions of a central bank and replaces the need for any
 central bank.