Rust

Minor: Add documentation explaining that initcap oly works for ASCII #4156

Job	Run time
Check License Header	2m 17s
cargo check	13m 19s
build with wasm-pack	3m 16s
cargo test (macos)	14m 32s
cargo test (macos-aarch64)	13m 26s
Check cargo fmt	46s
cargo test (win64)	1h 17m 47s
Verify MSRV (Min Supported Rust Version)	9m 21s
Verify Vendored Code	49s
cargo test datafusion-cli (amd64)	4m 49s
cargo examples (amd64)	14m 47s
cargo test (amd64)	12m 10s
cargo test doc (amd64)	17m 36s
cargo doc	4m 27s
check Cargo.toml formatting	2m 52s
clippy	6m 58s
check configs.md and ***_functions.md is up-to-date	3m 39s
cargo test hash collisions (amd64)	29m 4s
Run sqllogictest with Postgres runner	4m 12s
cargo test pyarrow (amd64)	3m 3s
verify benchmark results (amd64)	14m 40s
	4h 13m 50s

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