docs: add more doctests, reduce minumum fuel usage #514

Job	Run time
Build wasm	2m 1s
Run tests on Linux x86 (stable)	56s
Run tests on Linux arm64	4m 9s
Run tests on Linux armv7	4m 12s
Run tests on Linux x86 (stable, no default features)	55s
Run tests on Linux x86 (nightly, no default features)	53s
Run tests on Linux x86 (nightly)	58s
Run tests on macOS arm64 (Apple M1)	1m 10s
	15m 14s

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