Update warning message per review

src/compiler/scala/tools/nsc/ast/parser/Parsers.scala

@@ -2817,7 +2817,7 @@ self =>
           }
         if (nme.isEncodedUnary(name) && vparamss.nonEmpty) {
           val tpeStr = if (restype.isEmpty) "" else s" : $restype"
-          def unaryMsg(what: String) = s"empty-paren (nilary) prefix unary operator is $what: instead, remove () to declare as `def ${name.decodedName}$tpeStr`"
+          def unaryMsg(what: String) = s"unary prefix operator definition with empty parameter list is $what: instead, remove () to declare as `def ${name.decodedName}$tpeStr`"
           vparamss match {
             case List(List()) =>
               if (currentRun.isScala3) syntaxError(nameOffset, unaryMsg("unsupported"))

test/files/neg/prefix-unary-nilary-deprecation.check

@@ -1,4 +1,4 @@
-prefix-unary-nilary-deprecation.scala:4: warning: empty-paren (nilary) prefix unary operator is deprecated: instead, remove () to declare as `def unary_~ : Foo`
+prefix-unary-nilary-deprecation.scala:4: warning: unary prefix operator definition with empty parameter list is deprecated: instead, remove () to declare as `def unary_~ : Foo`
   def unary_~() : Foo = this
       ^
 prefix-unary-nilary-deprecation.scala:10: warning: Auto-application to `()` is deprecated. Supply the empty argument list `()` explicitly to invoke method unary_~,

test/files/neg/prefix-unary-nilary-removal.check

@@ -1,4 +1,4 @@
-prefix-unary-nilary-removal.scala:4: error: empty-paren (nilary) prefix unary operator is unsupported: instead, remove () to declare as `def unary_~ : Foo`
+prefix-unary-nilary-removal.scala:4: error: unary prefix operator definition with empty parameter list is unsupported: instead, remove () to declare as `def unary_~ : Foo`
   def unary_~() : Foo = this
       ^
 1 error