fix(#16610): warn Scaladoc on multiple cases

Before this commit, the compiler ignored Scaladoc comment on multiple
enum cases without warning.

This is partly expected because the case to which the doc is attached is ambiguous,
but we should at least warn users that the comment is ignored by compiler due to
ambiguity and they should take an action if they want the doc to be displayed.

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -3955,6 +3955,13 @@ object Parsers {
         if (in.token == COMMA) {
           in.nextToken()
           val ids = commaSeparated(() => termIdent())
+          in.getDocComment(start).foreach: comm =>
+            warning(
+              em"""Ambiguous Scaladoc comment on multiple cases is ignored.
+                   |Remove the comment or make separate cases and add Scaladoc comments to each of them.""",
+              comm.span.start
+            )
+
           PatDef(mods1, id :: ids, TypeTree(), EmptyTree)
         }
         else {

tests/warn/i16610.check

@@ -0,0 +1,5 @@
+-- Warning: tests/warn/i16610.scala:11:2 -------------------------------------------------------------------------------
+11 |  /** // warn
+   |  ^
+   |  Ambiguous Scaladoc comment on multiple cases is ignored.
+   |  Remove the comment or make separate cases to add Scaladoc comments to each of them.

tests/warn/i16610.scala

@@ -0,0 +1,15 @@
+/**
+ * Description of enum
+ */
+enum MyEnum {
+
+  /**
+   * Description of case 1
+   */
+  case MyCase1
+
+  /** // warn
+   * Description of case 2 and 3
+   */
+  case MyCase2, MyCase3
+}