Minor: Add documentation for `cot` #33963

Job	Run time
Check cargo fmt	42s
cargo check	7m 54s
build with wasm-pack	2m 45s
cargo test (macos)	12m 3s
cargo test (macos-aarch64)	10m 8s
Verify MSRV (Min Supported Rust Version)	8m 25s
Verify Vendored Code	57s
cargo test (win64)	53m 15s
cargo test datafusion-cli (amd64)	4m 2s
cargo test doc (amd64)	15m 18s
cargo examples (amd64)	13m 31s
cargo test (amd64)	10m 37s
clippy	6m 17s
cargo doc	4m 11s
cargo test hash collisions (amd64)	17m 15s
Run sqllogictest with Postgres runner	3m 38s
cargo test pyarrow (amd64)	2m 45s
verify benchmark results (amd64)	12m 18s
check Cargo.toml formatting	3m 5s
check configs.md and ***_functions.md is up-to-date	3m 11s
	3h 12m 17s

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