Fix doc warnings in xilem_core, xilem_web #950

Job	Run time
cargo check (msrv) (windows-latest)	3m 6s
cargo check (aarch64-android)	1m 33s
cargo check (msrv) (wasm32)	30s
cargo check (msrv) (macos-latest)	1m 12s
cargo clippy (windows-latest)	5m 18s
cargo check (msrv) (ubuntu-latest)	1m 33s
cargo clippy (macos-latest)	2m 21s
cargo clippy (wasm32)	1m 41s
cargo doc	1m 25s
cargo clippy (ubuntu-latest)	3m 45s
formatting	25s
cargo test (windows-latest)	5m 7s
cargo test (wasm32)	43s
typos	4s
cargo test (macos-latest)	1m 43s
cargo test (ubuntu-latest)	4m 11s
	34m 37s

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