Refine diagnostics for nullability migration warnings

#KT-24911 Fixed

compiler/testData/cli/jvm/jsr305/Annotated.java

@@ -5,4 +5,9 @@ public class Annotated {
 
     public void bar(@MyMigrationNonnull String x) {
     }
-}
+
+    @MyNullable
+    public String nullable() {
+        return null;
+    }
+}

compiler/testData/cli/jvm/jsr305/MyNullable.java

@@ -0,0 +1,15 @@
+import javax.annotation.*;
+import java.lang.annotation.Documented;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+
+import javax.annotation.meta.TypeQualifierNickname;
+import javax.annotation.meta.When;
+
+@Documented
+@TypeQualifierNickname
+@Nonnull(when = When.MAYBE)
+@Retention(RetentionPolicy.RUNTIME)
+public @interface MyNullable {
+
+}