Stabilize EnumEntries stdlib signature

* Also, mention implementation details of EnumEntries to enable safe uses of a 'EnumEntries' as a set
* Fix testdata for opt-in marker in enum entries

^KT-58548 fixed

Merge-request: KT-MR-10037
Merged-by: Vsevolod Tolstopyatov <qwwdfsad@gmail.com>

compiler/testData/diagnostics/tests/enum/entries/entriesUnsupported.kt

@@ -7,5 +7,5 @@ enum class Foo {
 }
 
 fun main() {
-    Foo.<!OPT_IN_USAGE_ERROR, UNSUPPORTED_FEATURE!>entries<!>
+    Foo.<!UNSUPPORTED_FEATURE!>entries<!>
 }