Document better -keypool as a look-ahead safety mechanism

If after a backup, an address is issued beyond the initial
keypool range and none of the addresses in this range
is seen onchain, if a wallet is restored from backup, even in
case of rescan, funds may be loss due to the look-ahead
buffer not being incremented and so restored wallet not detecting
onchain out-of-range address as derived from its seed.

This scenario is theoretically unavoidable due to the requirement
of the keypool to have a max size. However, given the default
keypool size, this is unlikely. Document better keypool size
implications to avoid user setting a too low value.

Author: Antoine Riard

src/wallet/init.cpp

@@ -46,7 +46,7 @@ void WalletInit::AddWalletOptions() const
 
     gArgs.AddArg("-fallbackfee=<amt>", strprintf("A fee rate (in %s/kB) that will be used when fee estimation has insufficient data. 0 to entirely disable the fallbackfee feature. (default: %s)",
                                                                CURRENCY_UNIT, FormatMoney(DEFAULT_FALLBACK_FEE)), ArgsManager::ALLOW_ANY, OptionsCategory::WALLET);
-    gArgs.AddArg("-keypool=<n>", strprintf("Set key pool size to <n> (default: %u)", DEFAULT_KEYPOOL_SIZE), ArgsManager::ALLOW_ANY, OptionsCategory::WALLET);
+    gArgs.AddArg("-keypool=<n>", strprintf("Set key pool size to <n> (default: %u). Warning: Smaller sizes may increase the risk of losing funds when restoring from an old backup, if none of the addresses in the original keypool have been used.", DEFAULT_KEYPOOL_SIZE), ArgsManager::ALLOW_ANY, OptionsCategory::WALLET);
     gArgs.AddArg("-maxtxfee=<amt>", strprintf("Maximum total fees (in %s) to use in a single wallet transaction; setting this too low may abort large transactions (default: %s)",
         CURRENCY_UNIT, FormatMoney(DEFAULT_TRANSACTION_MAXFEE)), ArgsManager::ALLOW_ANY, OptionsCategory::DEBUG_TEST);
     gArgs.AddArg("-mintxfee=<amt>", strprintf("Fees (in %s/kB) smaller than this are considered zero fee for transaction creation (default: %s)",

src/wallet/scriptpubkeyman.h

@@ -72,6 +72,11 @@ std::vector<CKeyID> GetAffectedKeys(const CScript& spk, const SigningProvider& p
  * keys (by default 1000) ahead of the last used key and scans for the
  * addresses of those keys.  This avoids the risk of not seeing transactions
  * involving the wallet's addresses, or of re-using the same address.
+ * In the unlikely case where none of the addresses in the `gap limit` are
+ * used on-chain, the look-ahead will not be incremented to keep
+ * a constant size and addresses beyond this range will not be detected by an
+ * old backup. For this reason, it is not recommended to decrease keypool size
+ * lower than default value.
  *
  * The HD-split wallet feature added a second keypool (commit: 02592f4c). There
  * is an external keypool (for addresses to hand out) and an internal keypool