doc(wallet): Add docs to explain the lookahead #6032

Job	Run time
Build Examples (example_cli)	32s
Check no_std	45s
Build Examples (example_bitcoind_rpc_polling)	38s
Build and test (stable, true, --no-default-features)	4m 20s
Build Examples (example_electrum)	56s
Build and test (stable, true, --all-features)	4m 21s
Check WASM	1m 18s
Build Examples (example_esplora)	55s
Build and test (1.63.0, --no-default-features)	5m 7s
Rust fmt	11s
Build Examples (wallet_electrum)	54s
Build and test (1.63.0, --all-features)	5m 28s
Build Examples (wallet_esplora_async)	58s
prepare	5s
Build Examples (wallet_esplora_blocking)	59s
Build Examples (wallet_rpc)	1m 9s
clippy_check	1m 21s
	29m 57s

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