Clarify that `SampleLayout::in_bounds_index` has unspecified behavior for invalid indices #973

Job	Run time
build_benchmarks	1m 19s
Fuzz targets (afl)	1m 51s
cargo-deny	25s
Fuzz targets (cargo-fuzz)	8m 35s
clippy	3m 1s
public_private_dependencies	25s
rustfmt	10s
test_avif	2m 10s
test_avif_decoding	1m 41s
test_big_endian	1m 30s
test_big_endian (default)	3m 19s
test_features	47s
test_toolchains (1.61.0)	2m 59s
verify_msrv	3m 51s
test_big_endian (webp)	1m 37s
test_toolchains (nightly)	3m 15s
test_big_endian (webp-encoder)	3m 7s
test_toolchains (beta)	3m 18s
test_features (default)	2m 6s
test_features (gif)	1m 0s
test_features (jpeg)	1m 7s
test_features (png)	59s
test_features (tiff)	1m 23s
test_features (ico)	1m 3s
test_features (pnm)	55s
test_features (tga)	1m 12s
test_features (webp)	1m 16s
test_features (bmp)	54s
test_features (hdr)	45s
test_features (dxt)	47s
test_features (dds)	52s
test_features (farbfeld)	1m 3s
test_features (openexr)	1m 15s
test_features (jpeg_rayon)	1m 10s
test_features (webp-encoder)	1m 22s
	1h 2m 29s

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