Rust

Add a few more comments #24

Job	Run time
cargo check	4m 10s
build with wasm-pack	2m 17s
cargo test (macos)	20m 41s
Verify Vendored Code	44s
cargo test (win64)	41m 39s
Check cargo fmt	27s
Verify MSRV (Min Supported Rust Version)	7m 34s
cargo test datafusion-cli (amd64)	3m 19s
cargo examples (amd64)	7m 48s
cargo test (amd64)	9m 22s
cargo test hash collisions (amd64)	8m 53s
cargo doc	3m 44s
clippy	4m 46s
cargo test doc (amd64)	10m 19s
Run sqllogictest with Postgres runner	3m 1s
cargo test pyarrow (amd64)	2m 39s
verify benchmark results (amd64)	9m 58s
check Cargo.toml formatting	1m 15s
check configs.md is up-to-date	2m 46s
	2h 25m 22s

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