Improve `FixedDecimal` documentation (#5242) #2091

Job	Run time
WASM Demo	2m 12s
Check Credentials for jobs that need JSON credentials	2s
bench-binsize	2m 51s
bench-memory (macos-latest)	3m 50s
bench-memory (ubuntu-latest)	3m 38s
bench-memory (windows-latest)	8m 17s
bench-perf (components/collections)	4m 36s
bench-perf (components/datetime)	4m 21s
bench-perf (components/locale)	2m 40s
bench-perf (components/locale_core)	5m 26s
bench-perf (components/plurals)	6m 10s
bench-perf (components/segmenter)	3m 41s
bench-perf (provider/blob)	3m 24s
bench-perf (utils/fixed_decimal)	6m 8s
bench-perf (utils/litemap)	2m 10s
bench-perf (utils/tinystr)	6m 40s
bench-perf (utils/writeable)	3m 36s
bench-perf (utils/zerotrie)	5m 47s
bench-perf (utils/zerovec)	8m 29s
Rust docs	5m 19s
Dart docs	1m 35s
TypeScript docs	37s
C++ docs	1m 51s
Deploy to GitHub Pages	17m 53s
	1h 51m 13s

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