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/entriesPropertyAsExtensionClash.fir.kt

@@ -12,7 +12,7 @@ enum class A {
 val A.Companion.entries: Int get() = 0
 
 fun test() {
-    A.<!OPT_IN_USAGE_ERROR!>entries<!>
+    A.entries
     A.Companion.entries
 
     with(A) {
@@ -26,4 +26,4 @@ fun test() {
 
     val aCompanion = A.Companion
     aCompanion.entries
-}
+}