Add comment so pub items are not removed

As suggested in rust-lang#115393, add a comment so someone doesn't clean these up.

compiler/rustc_errors/src/lib.rs

@@ -55,6 +55,8 @@ use std::num::NonZeroUsize;
 use std::panic;
 use std::path::{Path, PathBuf};
 
+// Used by external projects such as `rust-gpu`.
+// See https://github.com/rust-lang/rust/pull/115393.
 pub use termcolor::{Color, ColorSpec, WriteColor};
 
 pub mod annotate_snippet_emitter_writer;