Minor: Improve documentation about optimizer #25399

Job	Run time
build with wasm-pack	2m 17s
cargo check	4m 9s
cargo test (macos)	17m 52s
Check cargo fmt	43s
cargo test (macos-aarch64)	9m 1s
Verify Vendored Code	50s
Verify MSRV (Min Supported Rust Version)	7m 1s
cargo test (win64)	42m 28s
cargo examples (amd64)	10m 53s
circular dependency check	2m 44s
cargo doc	3m 46s
cargo test (amd64)	9m 3s
cargo test datafusion-cli (amd64)	3m 32s
check Cargo.toml formatting	2m 57s
cargo test doc (amd64)	11m 31s
clippy	5m 6s
check configs.md is up-to-date	2m 37s
cargo test hash collisions (amd64)	9m 4s
Run sqllogictest with Postgres runner	3m 12s
cargo test pyarrow (amd64)	2m 37s
verify benchmark results (amd64)	10m 25s
	2h 41m 48s

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