Improve explain message for abstract decl lacking return type

Author: ljdelight

src/dotty/tools/dotc/reporting/diagnostic/messages.scala

@@ -487,10 +487,10 @@ object messages {
     val kind = "Syntax"
     val msg = "missing return type"
     val explanation =
-      hl"""The return type is missing and needs to be clarified. For example, `area` returns a ${"Double"}:
+      hl"""An abstract declaration must have a return type. For example:
         |
         |trait Shape {
-        |  def area: Double
-        |}""".stripMargin
+        |  def area: Double // abstract declaration returning a ${"Double"}
+        |}"""".stripMargin
   }
 }