Document special value semantics. #6

Job	Run time
Clippy	15s
Documentation	22s
Test (rust_decimal, --features rust_decimal)	20s
Format	13s
Minimum Supported Rust Version	15s
Test (default features)	22s
Test (all features, --all-features)	28s
Test (bigdecimal, --features bigdecimal)	19s
	2m 34s

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