Document how `BoxFuture`s / `BoxStream`s are often made #2843

Job	Run time
cargo clippy	24s
cargo +1.36 build (futures-{core, io, sink})	32s
cargo bench	52s
cargo +stable build	41s
cargo doc	17s
cargo +beta build	38s
cargo hack check --feature-powerset	2m 18s
cargo +nightly build	35s
cargo fmt	8s
cargo minimal-versions build	29s
cargo miri test	12m 31s
cargo build --target thumbv7m-none-eabi	27s
cargo test -Z sanitizer=address	3m 36s
cargo build --target thumbv6m-none-eabi	27s
cargo test (ubuntu-latest)	2m 54s
cargo +1.56 build	1m 21s
cargo test -Z sanitizer=memory	4m 56s
cargo test (macos-latest)	6m 23s
cargo test -Z sanitizer=thread	2m 33s
cargo test (windows-latest)	4m 55s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	3m 28s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	6m 34s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	8m 8s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	3m 47s
	1h 8m 54s

Provide feedback