Minor: improve documentation for IsNotNull, DISTINCT, etc #2410

Job	Run time
cargo check	6m 45s
build with wasm-pack	1m 47s
Verify Vendored Code	1m 3s
cargo test (mac)	22m 39s
cargo test (win64)	18m 38s
Check cargo fmt	41s
Verify MSRV	6m 27s
cargo examples (amd64)	4m 38s
cargo doc	2m 48s
cargo test datafusion-cli (amd64)	3m 0s
cargo test (amd64)	19m 31s
cargo test doc (amd64)	14m 45s
Run sqllogictest with Postgres runner	5m 54s
verify benchmark results (amd64)	19m 3s
cargo test pyarrow (amd64)	2m 2s
check Cargo.toml formatting	2m 14s
clippy	4m 15s
check configs.md is up-to-date	3m 29s
cargo test hash collisions (amd64)	8m 28s
	2h 28m 7s

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