CH04::privkey formats: add sidebar about format relevancy

Most software today doesn't export or import private keys, so add a
sidebar noting that this section is mainly for historical reasons.

ch04.asciidoc

@@ -946,6 +946,31 @@ internally in software and rarely shown to users. The WIF is used for
 import/export of keys between wallets and often used in QR code
 (barcode) representations of private keys.
 
+.Modern relevancy of private key formats
+****
+Early Bitcoin wallet software generated one or more independent private
+keys when a new user wallet was initialized.  When the initial set of
+keys had all been used, the wallet might generate additional private
+keys.  Individual private keys could be exported or imported.  Any time
+new private keys were generated or imported, a new backup of the wallet
+needed to be created.
+
+Later Bitcoin wallets began using deterministic wallets where all
+private keys are generated from a single seed value.  These wallets only
+ever need to be backed up once for typical onchain use.  However, if a
+user exports a single private key from one of these wallets and an
+attacker acquires that key plus some non-private data about the wallet,
+they can potentially derive any private key in the wallet--allowing the
+attacker to steal all of the wallet funds.  Additionally, keys cannot be
+imported into deterministic wallets.  This means almost no modern
+wallets support the ability to export or import an individual key.  The
+information in this section is mainly of interest to anyone needing
+compatibility with early Bitcoin wallets.
+
+For more information, see <<hd_wallets>>.
+
+****
+
 [[table_4-2]]
 .Private key representations (encoding formats)
 [options="header"]