CH05::Intro: re-title and re-introduce

- Retitle from "wallets" to "wallet recovery".  The existing chapter is
  entirely about generating keys in a way that can be recovered after a
  data loss.  I worry that calling this chapter "wallets" results in ignoring
  many other aspects of wallet design, such as how they scan for
  transactions (important for privacy) and how they sign (important for
  security and wallet interoperation).

- Re-introduce the chapter, given the changed name.

ch05.asciidoc

@@ -1,20 +1,21 @@
 [[ch05_wallets]]
-== Wallets
-
-((("wallets", "defined")))The word "wallet" is used to describe a few
-different things in bitcoin.
-
-At a high level, a wallet is an application that serves as the primary
-user interface. The wallet controls access to a user's money, managing
-keys and addresses, tracking the balance, and creating and signing
-transactions.
-
-More narrowly, from a programmer's perspective, the word "wallet" refers
-to the data structure used to store and manage a user's keys.
-
-In this chapter we will look at the second meaning, where wallets are
-containers for private keys, usually implemented as structured files or
-simple databases.
+== Wallet Recovery
+
+Creating pairs of private and public keys is a crucial part of allowing
+Bitcoin wallets to receive and spend bitcoins.  But losing access to a
+private key can make it impossible for anyone to ever spend the bitcoins
+received to the corresponding public key.  Wallet and protocol
+developers over the years have worked to design systems that allow users
+to recover access to their bitcoins after a problem without compromising
+security the rest of the time.
+
+In this chapter, we'll examine some of the different methods employed by
+wallets to prevent the loss of data from becoming a loss of money.
+Some solutions have almost no downsides and are universally adopted by
+modern wallets.  We'll simply recommend those solutions as best
+practices.  Other solutions have both advantages and disadvantages,
+leading different wallet authors to make different tradeoffs.
+In those cases, we'll describe the various options available.
 
 === Wallet Technology Overview