Note allowed --emit metadata in rust doc

[l4l]

as it in the source:

sccache/src/compiler/rust.rs

Lines 213 to 214 in 69be532

	static ALLOWED_EMIT: Lazy<HashSet<&'static str>> =
	Lazy::new(|| ["link", "metadata", "dep-info"].iter().copied().collect());

[Xuanwo]

Thanks!