Narrow and document the `getrandom` `js` feature usage. #702

Job	Run time
cargo clippy (wasm32)	41s
cargo check (msrv) (windows-latest)	1m 2s
cargo check (msrv) (wasm32)	32s
cargo check (msrv) (macos-latest)	50s
cargo doc	17s
cargo clippy (windows-latest)	1m 20s
cargo check (msrv) (ubuntu-latest)	46s
cargo clippy (macos-latest)	53s
formatting	22s
cargo clippy (ubuntu-latest)	54s
cargo test (windows-latest)	49s
cargo test (wasm32)	25s
cargo test (macos-latest)	28s
typos	4s
cargo test (ubuntu-latest)	22s
	9m 45s

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