Update WebPDecoder doc comments #1593

Job	Run time
build_benchmarks	47s
Fuzz targets (afl)	2m 36s
Fuzz targets (cargo-fuzz)	56s
cargo-deny	18s
cargo-semver-checks	22s
public_private_dependencies	24s
clippy	2m 40s
rustfmt	4s
test_avif_decoding	1m 24s
test_other_archs (powerpc-unknown-linux-gnu)	2m 38s
test_toolchains (1.67.1)	34s
test_other_archs (i686-unknown-linux-gnu)	1m 32s
test_features	36s
test_toolchains (nightly)	1m 2s
test_features (default)	1m 46s
test_toolchains (beta)	1m 35s
test_features (rayon)	32s
test_features (avif)	56s
test_features (bmp)	32s
test_features (dds)	28s
test_features (exr)	46s
test_features (ff)	32s
test_features (gif)	40s
test_features (hdr)	29s
test_features (ico)	41s
test_features (jpeg)	35s
test_features (png)	50s
test_features (pnm)	29s
test_features (qoi)	27s
test_features (tga)	32s
test_features (tiff)	35s
test_features (webp)	38s
	28m 56s

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