Document cancel-safety of FuturesUnordered::poll_next #2906

Job	Run time
cargo +1.36 build (futures-{core, io, sink})	53s
cargo +stable build	56s
cargo bench	52s
cargo +beta build	44s
cargo hack check --feature-powerset	2m 25s
cargo +nightly build	40s
cargo minimal-versions build	30s
cargo miri test	12m 50s
cargo build --target thumbv7m-none-eabi	31s
cargo test (ubuntu-latest)	2m 38s
cargo build --target thumbv6m-none-eabi	27s
cargo test (macos-latest)	6m 38s
cargo +1.56 build	1m 28s
cargo test (windows-latest)	4m 52s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	3m 55s
cargo clippy	25s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	6m 41s
cargo doc	21s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	6m 55s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	3m 39s
cargo fmt	16s
cargo test -Z sanitizer=address	3m 13s
cargo test -Z sanitizer=memory	4m 0s
cargo test -Z sanitizer=thread	2m 28s
	1h 8m 17s

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