docs(wallet): reword the `next_unused_address` doc #6499

Job	Run time
Check no_std	27s
Check WASM	1m 4s
Rust fmt	7s
Build and test (stable, true, --no-default-features --features miniscript/no-std)	2m 18s
Build and test (stable, true, --all-features)	4m 8s
prepare	4s
Build and test (1.63.0, --no-default-features --features miniscript/no-std)	2m 27s
Build and test (1.63.0, --all-features)	5m 20s
Build & Test Examples (example_cli)	27s
Build & Test Examples (example_bitcoind_rpc_polling)	48s
Build & Test Examples (example_electrum)	56s
Build & Test Examples (example_esplora)	49s
Build & Test Examples (example_wallet_electrum)	53s
Build & Test Examples (example_wallet_esplora_async)	1m 5s
Build & Test Examples (example_wallet_esplora_blocking)	49s
Build & Test Examples (example_wallet_rpc)	34s
clippy_check	1m 11s
	23m 27s

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