Minor: Doc and organize fields in `struct ExternalSorter` #35009

Job	Run time
Check License Header	2m 29s
cargo check	8m 1s
build with wasm-pack	2m 47s
Check cargo fmt	45s
cargo test (macos)	10m 53s
cargo test (macos-aarch64)	10m 54s
Verify MSRV (Min Supported Rust Version)	8m 48s
Verify Vendored Code	52s
cargo test (win64)	1h 8m 1s
cargo test datafusion-cli (amd64)	4m 20s
cargo examples (amd64)	13m 52s
clippy	6m 16s
check Cargo.toml formatting	3m 4s
cargo doc	4m 36s
cargo test hash collisions (amd64)	17m 45s
cargo test (amd64)	11m 42s
cargo test doc (amd64)	16m 35s
Run sqllogictest with Postgres runner	3m 48s
cargo test pyarrow (amd64)	3m 0s
verify benchmark results (amd64)	12m 35s
check configs.md and ***_functions.md is up-to-date	3m 19s
	3h 34m 22s

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