Add `external` option to `unused-noqa` documentation #20638

Job	Run time
cargo clippy	2m 12s
benchmarks	12m 25s
cargo fuzz	5m 6s
cargo fmt	18s
cargo test (wasm)	1m 28s
cargo test \| ubuntu-latest	5m 28s
cargo udeps	3m 20s
Determine changes	11s
cargo test \| windows-latest	10m 0s
mkdocs	2m 7s
pre-commit	1m 36s
test scripts	1m 11s
python package	1m 36s
Formatter ecosystem and progress checks	0s
test ruff-lsp	26s
ecosystem	7m 1s
	54m 25s

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