add comment to __findMutatingUnsafe

Author: mkandil

stdlib/public/Cxx/CxxDictionary.swift

@@ -28,6 +28,9 @@ public protocol CxxDictionary<Key, Value> {
   /// Do not implement this function manually in Swift.
   func __findUnsafe(_ key: Key) -> RawIterator
 
+  /// Do not implement this function manually in Swift.
+  func __beginUnsafe() -> RawIterator
+  
   /// Do not implement this function manually in Swift.
   mutating func __findMutatingUnsafe(_ key: Key) -> RawMutableIterator