ReadBytesExt: document the danger of reading after an error #1699

Job	Run time
miri	1m 18s
test (pinned, ubuntu-latest, 1.60.0)	2m 6s
rustfmt	4s
test (stable, ubuntu-latest, stable)	48s
test (beta, ubuntu-latest, beta)	33s
test (nightly, ubuntu-latest, nightly)	51s
test (macos, macos-latest, stable)	26s
test (win-msvc, windows-latest, stable)	2m 3s
test (win-gnu, windows-latest, stable-x86_64-gnu)	1m 26s
test (stable-x86, ubuntu-latest, stable, i686-unknown-linux-gnu)	43s
test (stable-aarch64, ubuntu-latest, stable, aarch64-unknown-linux-gnu)	46s
test (stable-powerpc64, ubuntu-latest, stable, powerpc64-unknown-linux-gnu)	46s
test (stable-s390x, ubuntu-latest, stable, s390x-unknown-linux-gnu)	1m 4s
	12m 54s

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