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/nameShadowingOfExternallyDefinedEntriesOn.fir.kt

@@ -9,7 +9,7 @@ enum class E {
     ;
 
     fun foo() {
-        <!OPT_IN_USAGE_ERROR!>entries<!>.<!UNRESOLVED_REFERENCE!>length<!>
+        entries.<!UNRESOLVED_REFERENCE!>length<!>
         pckg.entries.length
     }
 }
@@ -20,7 +20,7 @@ class A {
 
         class B {
             fun foo() {
-                <!OPT_IN_USAGE_ERROR!>entries<!>.<!UNRESOLVED_REFERENCE!>length<!>
+                entries.<!UNRESOLVED_REFERENCE!>length<!>
                 pckg.entries.length
             }
         }