Address the PR tasks

Author: maseev

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

@@ -1842,7 +1842,9 @@ object messages {
   case class OnlyFunctionsCanBeFollowedByUnderscore(pt: Type)(implicit ctx: Context)
     extends Message(OnlyFunctionsCanBeFollowedByUnderscoreID) {
     val kind = "Syntax"
-    val msg = hl"not a function: $pt; cannot be followed by `_'"
-    val explanation = ""
+    val msg = hl"Not a function: $pt: cannot be followed by ${"_"}"
+    val explanation =
+      hl"""The syntax ${"x _"} is no longer supported if ${"x"} is not a function.
+          |To convert to a function value, you need to explicitly write ${"() => x"}"""
   }
 }

compiler/test/dotty/tools/dotc/reporting/ErrorMessagesTests.scala

@@ -1066,9 +1066,7 @@ class ErrorMessagesTests extends ErrorMessagesTest {
       implicit val ctx: Context = ictx
 
       assertMessageCount(1, messages)
-
       val OnlyFunctionsCanBeFollowedByUnderscore(pt) :: Nil = messages
-
       assertEquals("String(n)", pt.show)
     }
 }