Document cancel-safety of FuturesUnordered::poll_next #2930

Job	Run time
cargo bench	43s
cargo +stable build	42s
cargo +1.36 build (futures-{core, io, sink})	29s
cargo clippy	15s
cargo +beta build	43s
cargo doc	19s
cargo hack check --feature-powerset	2m 29s
cargo +nightly build	37s
cargo fmt	6s
cargo minimal-versions build	31s
cargo miri test	13m 46s
cargo test -Z sanitizer=address	3m 18s
cargo build --target thumbv7m-none-eabi	26s
cargo test -Z sanitizer=memory	4m 9s
cargo test (ubuntu-latest)	2m 53s
cargo build --target thumbv6m-none-eabi	27s
cargo +1.56 build	59s
cargo test -Z sanitizer=thread	2m 33s
cargo test (macos-latest)	5m 41s
cargo test (windows-latest)	4m 50s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	3m 42s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	6m 23s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	6m 50s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	3m 29s
	1h 6m 20s

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