Rust

Add documentation for `SHOW FUNCTIONS` #4205

Job	Run time
Check License Header	2m 10s
cargo check	13m 35s
build with wasm-pack	3m 8s
cargo test (macos)	12m 38s
Check cargo fmt	32s
cargo test (macos-aarch64)	11m 32s
Verify MSRV (Min Supported Rust Version)	9m 13s
Verify Vendored Code	42s
cargo test (amd64)	12m 28s
cargo test datafusion-cli (amd64)	4m 36s
clippy	7m 0s
cargo doc	4m 21s
cargo test doc (amd64)	17m 32s
cargo examples (amd64)	14m 35s
Run sqllogictest with Postgres runner	4m 56s
cargo test pyarrow (amd64)	3m 5s
verify benchmark results (amd64)	14m 59s
check configs.md and ***_functions.md is up-to-date	3m 55s
cargo test hash collisions (amd64)	28m 55s
check Cargo.toml formatting	2m 52s
	2h 52m 44s

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