Use () to indicate a missing blockchain

So that:
1. There are no runtime errors
2. There less type annotations needed
3. Less traits and stuff to document

src/wallet/address_validator.rs

@@ -66,7 +66,7 @@
 //! }
 //!
 //! let descriptor = "wpkh(tpubD6NzVbkrYhZ4Xferm7Pz4VnjdcDPFyjVu5K4iZXQ4pVN8Cks4pHVowTBXBKRhX64pkRyJZJN5xAKj4UDNnLPb5p2sSKXhewoYx5GbTdUFWq/*)";
-//! let mut wallet: OfflineWallet<_> = Wallet::new_offline(descriptor, None, Network::Testnet, MemoryDatabase::default())?;
+//! let mut wallet = Wallet::new_offline(descriptor, None, Network::Testnet, MemoryDatabase::default())?;
 //! wallet.add_address_validator(Arc::new(PrintAddressAndContinue));
 //!
 //! let address = wallet.get_new_address()?;