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/signer.rs

@@ -79,7 +79,7 @@
 //! let custom_signer = CustomSigner::connect();
 //!
 //! 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_signer(
 //!     KeychainKind::External,
 //!     Fingerprint::from_str("e30f11b8").unwrap().into(),