Add documenation to the emit_for message

src/libsyntax/diagnostic.rs

@@ -207,6 +207,10 @@ impl Handler {
         if lvl == Warning && !self.can_emit_warnings { return }
         self.emit.borrow_mut().emit(cmsp, msg, Some(code), lvl);
     }
+    /// Function to emit a message on a level ment to accompany a message on another level.
+    /// If a warning is issued with a note, the note may be emmitted using emit_for with
+    /// a lvl warning of Note and a for_lvl parameter of Warning. The other parameters are the same
+    /// as in the emit function.
     pub fn emit_for(&self,
                 cmsp: Option<(&codemap::CodeMap, Span)>,
                 msg: &str,