Improve docstring of `Ui::new_child` #12797

Job	Run time
Format + check + test	5m 25s
Check wasm32+atomics	1m 30s
android	19s
cargo-deny aarch64-apple-darwin	40s
Check wasm32 + wasm-bindgen	2m 36s
ios	21s
cargo-deny aarch64-linux-android	40s
Check Windows	1m 36s
cargo-deny i686-pc-windows-gnu	38s
cargo-deny i686-pc-windows-msvc	35s
cargo-deny i686-unknown-linux-gnu	37s
cargo-deny wasm32-unknown-unknown	37s
cargo-deny x86_64-apple-darwin	39s
cargo-deny x86_64-pc-windows-gnu	37s
cargo-deny x86_64-pc-windows-msvc	37s
cargo-deny x86_64-unknown-linux-gnu	37s
cargo-deny x86_64-unknown-linux-musl	40s
cargo-deny x86_64-unknown-redox	44s
	19m 28s

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