[Docs] Clarifies Identifiable requirements

[SR-12528](https://bugs.swift.org/browse/SR-12528)
rdar://62201744

Author: Kyle Macomber

stdlib/public/core/Identifiable.swift

@@ -10,14 +10,42 @@
 //
 //===----------------------------------------------------------------------===//
 
-/// A class of types whose instances hold the value of an entity with stable identity.
+/// A class of types whose instances hold the value of an entity with stable
+/// identity.
+///
+/// Use the `Identifiable` protocol to provide a stable notion of identity to a 
+/// class or value type. For example, you could define a `User` type with an `id`
+/// property that is stable across your app and your app's database storage. 
+/// You could use the `id` property to identify a particular user even if other 
+/// data fields change, such as the user's name.
+///
+/// `Identifiable` leaves the duration and scope of the identity unspecified.
+/// Identities could be any of the following:
+///
+/// - Guaranteed always unique (e.g. UUIDs).
+/// - Persistently unique per environment (e.g. database record keys).
+/// - Unique for the lifetime of a process (e.g. global incrementing integers).
+/// - Unique for the lifetime of an object (e.g. object identifiers).
+/// - Unique within the current collection (e.g. collection index).
+///
+/// It is up to both the conformer and the receiver of the protocol to document
+/// the nature of the identity.
+///
+/// Conforming to the Identifiable Protocol
+/// =======================================
+///
+/// `Identifiable` provides a default implementation for class types (using
+/// `ObjectIdentifier`), which 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.
 @available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
 public protocol Identifiable {
 
-  /// A type representing the stable identity of the entity associated with `self`.
+  /// A type representing the stable identity of the entity associated with
+  /// an instance.
   associatedtype ID: Hashable
 
-  /// The stable identity of the entity associated with `self`.
+  /// The stable identity of the entity associated with this instance.
   var id: ID { get }
 }