[Docs] Clarifies ObjectIdentifier guarantees (swiftlang#31472)

[SR-13564](https://bugs.swift.org/browse/SR-13564)
rdar://69169351

Author: sunbohong

stdlib/public/core/ObjectIdentifier.swift

@@ -15,6 +15,11 @@
 /// In Swift, only class instances and metatypes have unique identities. There
 /// is no notion of identity for structs, enums, functions, or tuples.
 @frozen // trivial-implementation
+/// 
+/// `ObjectIdentifier` is only guaranteed to remain unique for the
+/// lifetime of an object. If an object has a stronger notion of identity, it
+/// may be appropriate to provide a custom implementation.
+
 public struct ObjectIdentifier {
   @usableFromInline // trivial-implementation
   internal let _value: Builtin.RawPointer