Clarify that `SampleLayout::in_bounds_index` has unspecified behavior for invalid indices #975

Job	Run time
test_big_endian	1m 43s
test_big_endian (default)	3m 5s
test_big_endian (webp)	1m 35s
test_avif	1m 57s
test_big_endian (webp-encoder)	3m 4s
test_features	57s
test_avif_decoding	2m 6s
test_features (default)	2m 23s
test_features (gif)	1m 0s
test_toolchains (1.61.0)	3m 19s
test_features (jpeg)	50s
test_toolchains (nightly)	2m 8s
build_benchmarks	1m 41s
test_features (png)	1m 13s
test_toolchains (beta)	1m 45s
test_features (tiff)	1m 10s
test_features (ico)	1m 5s
Fuzz targets (cargo-fuzz)	7m 25s
Fuzz targets (afl)	1m 56s
test_features (pnm)	1m 9s
test_features (tga)	1m 4s
clippy	3m 9s
public_private_dependencies	26s
test_features (webp)	1m 1s
test_features (bmp)	1m 9s
rustfmt	11s
cargo-deny	35s
test_features (hdr)	55s
verify_msrv	3m 54s
test_features (dxt)	1m 8s
test_features (dds)	56s
test_features (farbfeld)	55s
test_features (openexr)	1m 13s
test_features (jpeg_rayon)	1m 0s
test_features (webp-encoder)	1m 37s
	1h 0m 44s

Provide feedback