Minor: Clarify documentation on PruningStatistics and make test match… #298

Job	Run time
cargo test (win64)	43m 10s
cargo test (macos)	21m 42s
cargo check	8m 20s
Check cargo fmt	36s
build with wasm-pack	2m 12s
cargo test (macos-aarch64)	8m 41s
Verify MSRV (Min Supported Rust Version)	6m 37s
Verify Vendored Code	50s
cargo test (amd64)	9m 2s
cargo doc	3m 34s
cargo test datafusion-cli (amd64)	3m 51s
circular dependency check	2m 31s
cargo test doc (amd64)	11m 39s
check Cargo.toml formatting	3m 0s
cargo examples (amd64)	10m 51s
clippy	5m 44s
check configs.md is up-to-date	2m 26s
cargo test hash collisions (amd64)	8m 36s
verify benchmark results (amd64)	10m 47s
Run sqllogictest with Postgres runner	3m 11s
cargo test pyarrow (amd64)	2m 42s
	2h 50m 2s

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