CH01::Mining: clarify that 99% of all bitcoins will be mined within a decade

Previous text mentioned all bitcoins would be mined by 2140, which is
correct but easily confuses people who don't understand exponential
decay into thinking a substantial number of bitcoins will continue to be
mined for a century.

FIXME.md

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+// FIXME
+- [ ] s/mnemonic/seed/

ch01.asciidoc

@@ -45,18 +45,21 @@ and clearing functions of a central bank and replaces the need for any
 central bank.
 
 The Bitcoin protocol includes built-in algorithms that regulate the
-mining function across the network. The difficulty of the processing
+mining function across the network. The difficulty of the mathematical
 task that miners must perform is adjusted dynamically so that, on
 average, someone succeeds every 10 minutes regardless of how many miners
 (and how much processing) are competing at any moment. The protocol also
-halves the rate at which new bitcoin is created every 4 years, and
+halves the rate at which new bitcoin is created approximately every 4 years, and
 limits the total number of bitcoin that will be created to a fixed total
 just below 21 million coins. The result is that the number of bitcoin in
-circulation closely follows an easily predictable curve that approaches
-21 million by the year 2140. Due to bitcoin's diminishing rate of
-issuance, over the long term, the Bitcoin currency is deflationary.
-Furthermore, bitcoin cannot be inflated by "printing" new money above
-and beyond the expected issuance rate.
+circulation closely follows an easily predictable curve where half of
+the remaining coins are added to circulation every four years.  By the
+time this book has been published for ten years, 99% of all bitcoins
+that will ever exist will have been issued.  Due to bitcoin's
+diminishing rate of issuance, over the long term, the Bitcoin currency
+is deflationary.  Furthermore, nobody can force you to accept bitcoin
+that has been inflated by creating new bitcoins above and beyond the
+expected issuance rate.
 
 Behind the scenes, Bitcoin is also the name of the protocol, a peer-to-peer network, and a distributed computing innovation. Bitcoin represents the culmination of decades of research in cryptography and distributed systems and includes four key innovations brought together in a unique and powerful combination. Bitcoin consists of: