docs: clarify the behavior of `Buf::chunk` (#717) #683

Job	Run time
loom	2m 4s
miri	10m 41s
cross (i686-unknown-linux-gnu)	44s
minrust	2m 7s
cross (armv7-unknown-linux-gnueabihf)	32s
nightly	3m 1s
rustfmt	3s
cross (powerpc-unknown-linux-gnu)	28s
cross (powerpc64-unknown-linux-gnu)	54s
cross (wasm32-unknown-unknown)	9s
stable (ubuntu-latest)	1m 15s
tsan	2m 21s
stable (macos-latest)	1m 5s
stable (windows-latest)	3m 40s
Publish Documentation	1m 55s
	30m 59s

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