docs: clarify the behavior of `Buf::chunk` #680

Job	Run time
miri	10m 47s
loom	1m 54s
cross (i686-unknown-linux-gnu)	39s
minrust	2m 20s
cross (armv7-unknown-linux-gnueabihf)	29s
nightly	3m 17s
cross (powerpc-unknown-linux-gnu)	30s
rustfmt	15s
cross (powerpc64-unknown-linux-gnu)	33s
stable (ubuntu-latest)	1m 9s
stable (macos-latest)	1m 9s
tsan	2m 17s
stable (windows-latest)	2m 39s
cross (wasm32-unknown-unknown)	17s
Publish Documentation	1m 38s
	29m 53s

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