message: doc: Update wallet files in files.md
diff: - [Multi-wallet environment](#multi-wallet-environment)
  + - [Berkeley DB database based wallets](#berkeley-db-database-based-wallets)
  +
  + - [SQLite database based wallets](#sqlite-database-based-wallets)
  +
  - [GUI settings](#gui-settings)
  - [Legacy subdirectories and files](#legacy-subdirectories-and-files)
  @@ -67,26 +71,36 @@ Subdirectory | File(s) | Description
  ## Multi-wallet environment
  -Wallets are Berkeley DB (BDB) databases:
  +Wallets are Berkeley DB (BDB) or SQLite databases.
  -Subdirectory | File(s) | Description
  --------------|-------------------|------------
  -`database/` | BDB logging files | Part of BDB environment; created at start and deleted on shutdown; a user *must keep it as safe* as personal wallet `wallet.dat`
  -`./` | `db.log` | BDB error file
  -`./` | `wallet.dat` | Personal wallet with keys and transactions. May be either a Berkeley DB or SQLite database file.
  -`./` | `.walletlock` | Wallet lock file
  -`./` | `wallet.dat-journal` | SQLite Rollback Journal file for `wallet.dat`. Usually created at start and deleted on shutdown. A user *must keep it as safe* as the `wallet.dat` file.
  -
  -1. Each user-defined wallet named "wallet_name" resides in `wallets/wallet_name/` subdirectory.
  +1. Each user-defined wallet named "wallet_name" resides in the `wallets/wallet_name/` subdirectory.
  2. The default (unnamed) wallet resides in `wallets/` subdirectory; if the latter does not exist, the wallet resides in the data directory.
  -3. A wallet database path can be specified by `-wallet` option.
  +3. A wallet database path can be specified with the `-wallet` option.
  4. `wallet.dat` files must not be shared across different node instances, as that can result in key-reuse and double-spends due the lack of synchronization between instances.
  5. Any copy or backup of the wallet should be done through a `backupwallet` call in order to update and lock the wallet, preventing any file corruption caused by updates during the copy.
  +
  +### Berkeley DB database based wallets
  +
  +Subdirectory | File(s) | Description
  +-------------|-------------------|-------------
  +`database/` | BDB logging files | Part of BDB environment; created at start and deleted on shutdown; a user *must keep it as safe* as personal wallet `wallet.dat`
  +`./` | `db.log` | BDB error file
  +`./` | `wallet.dat` | Personal wallet (a BDB database) with keys and transactions
  +`./` | `.walletlock` | BDB wallet lock file
  +
  +### SQLite database based wallets
  +
  +Subdirectory | File | Description
  +-------------|----------------------|-------------
  +`./` | `wallet.dat` | Personal wallet (a SQLite database) with keys and transactions
  +`./` | `wallet.dat-journal` | SQLite Rollback Journal file for `wallet.dat`. Usually created at start and deleted on shutdown. A user *must keep it as safe* as the `wallet.dat` file.
  +
  +
  ## GUI settings
  `peercoin-qt` uses [`QSettings`](https://doc.qt.io/qt-5/qsettings.html) class; this implies platform-specific [locations where application settings are stored](https://doc.qt.io/qt-5/qsettings.html#locations-where-application-settings-are-stored).
  