Docs: Improve the documentation on `ScalarValue` #20729

Job	Run time
Check cargo fmt	34s
cargo check	2m 35s
build with wasm-pack	1m 54s
cargo test (mac)	14m 25s
Verify MSRV	6m 11s
Verify Vendored Code	36s
cargo test (win64)	30m 16s
cargo test hash collisions (amd64)	8m 13s
clippy	4m 18s
check configs.md is up-to-date	2m 22s
check Cargo.toml formatting	1m 12s
cargo doc	2m 58s
cargo test (amd64)	8m 41s
cargo test datafusion-cli (amd64)	2m 52s
cargo test doc (amd64)	9m 56s
cargo examples (amd64)	6m 12s
Run sqllogictest with Postgres runner	3m 8s
cargo test pyarrow (amd64)	2m 13s
verify benchmark results (amd64)	9m 40s
	1h 58m 16s

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