wallet/docs: SQLite is the default backend, not experimental

Drop the "experimental" label from the wallet code and docs now that
SQLite is the default backend for new wallets, is coin-proven, and carries
encrypt, backup, atomic seed writes, and the storage diagnostics.

- main_gui.cpp: -help text, the unknown-backend error (SQLite is no longer
  "experimental" and Berkeley DB is no longer "the default"), and the
  startup banner ("Wallet backend: SQLite").
- walletcmd.h: the export command comment.
- docs/storage.md: rewrite the SQLite section from staging-only ("runtime
  still opens Berkeley DB", "no runtime backend flag exists yet") to the
  shipped state -- default backend, conversion wizard, per-backend ops --
  and reframe the migration order as shipped in v1.2.20.

1 files changed, 3 insertions, 3 deletions

diff --git a/src/walletcmd.h b/src/walletcmd.h
index 38cbfdb..7013d44 100644
--- a/src/walletcmd.h
+++ b/src/walletcmd.h
@@ -70,9 +70,9 @@ int CmdWalletStorageAudit(const std::string& strJsonOut = "");
 // or migration-unsafe mix of storage records.
 int CmdWalletStorageCheck();
 
-// Diagnostic/migration staging: copy every raw wallet.dat record into an
-// experimental SQLite key/value store, preserving serialized bytes. This does
-// not change which backend the runtime wallet opens.
+// Copy every raw wallet.dat record into a SQLite key/value store, preserving
+// serialized bytes. A migration/scripting tool; it does not change which backend
+// the runtime wallet opens.
 int CmdWalletSQLiteExport(const std::string& strDest);
 
 // Export the currently-loaded wallet to a SQLite file (shared by the command