Add some comments to Emitter.

There are three functions only used for the JSON format.

Author: nnethercote

compiler/rustc_errors/src/emitter.rs

@@ -196,13 +196,15 @@ pub trait Emitter: Translate {
     fn emit_diagnostic(&mut self, diag: &Diagnostic);
 
     /// Emit a notification that an artifact has been output.
-    /// This is currently only supported for the JSON format,
-    /// other formats can, and will, simply ignore it.
+    /// Currently only supported for the JSON format.
     fn emit_artifact_notification(&mut self, _path: &Path, _artifact_type: &str) {}
 
+    /// Emit a report about future breakage.
+    /// Currently only supported for the JSON format.
     fn emit_future_breakage_report(&mut self, _diags: Vec<Diagnostic>) {}
 
-    /// Emit list of unused externs
+    /// Emit list of unused externs.
+    /// Currently only supported for the JSON format.
     fn emit_unused_externs(
         &mut self,
         _lint_level: rustc_lint_defs::Level,