Document cancel-safety of StreamExt::next #2905

Job	Run time
cargo clippy	24s
cargo +1.36 build (futures-{core, io, sink})	54s
cargo bench	57s
cargo +stable build	41s
cargo doc	18s
cargo +beta build	42s
cargo hack check --feature-powerset	2m 22s
cargo fmt	14s
cargo +nightly build	36s
cargo minimal-versions build	30s
cargo miri test	13m 1s
cargo build --target thumbv7m-none-eabi	29s
cargo test -Z sanitizer=address	3m 9s
cargo build --target thumbv6m-none-eabi	32s
cargo test -Z sanitizer=memory	3m 46s
cargo test (ubuntu-latest)	2m 47s
cargo +1.56 build	1m 27s
cargo test -Z sanitizer=thread	2m 32s
cargo test (macos-latest)	5m 58s
cargo test (windows-latest)	4m 47s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	3m 31s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	7m 6s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	7m 2s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	3m 39s
	1h 7m 24s

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