[stdlib] Add post-nil guarantee for IteratorProtocol.next() to always return nil

stdlib/public/core/StringUnicodeScalarView.swift

@@ -162,11 +162,8 @@ extension String {
         }
       }
 
-      /// Advance to the next element and return it, or `nil` if no next
-      /// element exists.
-      ///
-      /// - Precondition: No preceding call to `self.next()` has returned
-      ///   `nil`.
+      /// Advance to the next element and return it, or `nil` if no next element
+      /// exists. Once `nil` has been returned, all subsequent calls return `nil`.
       public mutating func next() -> UnicodeScalar? {
         var result: UnicodeDecodingResult
         if _baseSet {