Make constraints on error message IDs clearer

Author: nicolasstucki

compiler/src/dotty/tools/dotc/reporting/ErrorMessageID.scala

@@ -1,13 +1,16 @@
 package dotty.tools.dotc.reporting
 
+//////////////////////////////////////////////////////////////////////////
+// IMPORTANT                                                            //
+// Only add new IDs at end of the enumeration list and never remove IDs //
+//////////////////////////////////////////////////////////////////////////
+
 /** Unique IDs identifying the messages, this will be used to reference documentation online.
  *
  *  @param isActive Whether or not the compile still emits this ErrorMessageID
  **/
 enum ErrorMessageID(val isActive: Boolean = true) extends java.lang.Enum[ErrorMessageID]:
 
-  // IMPORTANT: Add new IDs only at the end and never remove IDs
-
   case NoExplanationID // errorNumber: -1
 
   case EmptyCatchOrFinallyBlockID extends ErrorMessageID(isActive = false) // errorNumber: 0