Explain the () argument to ErrorGuaranteed.

Author: nnethercote

compiler/rustc_span/src/lib.rs

@@ -2247,6 +2247,9 @@ where
 
 /// Useful type to use with `Result<>` indicate that an error has already
 /// been reported to the user, so no need to continue checking.
+///
+/// The `()` field is necessary: it is non-`pub`, which means values of this
+/// type cannot be constructed outside of this crate.
 #[derive(Clone, Copy, Debug, Hash, PartialEq, Eq, PartialOrd, Ord)]
 #[derive(HashStable_Generic)]
 pub struct ErrorGuaranteed(());