stdlib: update comments for the array buffer endCowMutation functions.

Author: eeckstein

stdlib/public/core/ArrayBuffer.swift

@@ -138,7 +138,7 @@ extension _ArrayBuffer {
 
   /// Puts the buffer in an immutable state.
   ///
-  /// - Precondition: The buffer must be mutable.
+  /// - Precondition: The buffer must be mutable or the empty array singleton.
   ///
   /// - Warning: After a call to `endCOWMutation` the buffer must not be mutated
   ///   until the next call of `beginCOWMutation`.

stdlib/public/core/ContiguousArrayBuffer.swift

@@ -697,7 +697,7 @@ internal struct _ContiguousArrayBuffer<Element>: _ArrayBufferProtocol {
 
   /// Puts the buffer in an immutable state.
   ///
-  /// - Precondition: The buffer must be mutable.
+  /// - Precondition: The buffer must be mutable or the empty array singleton.
   ///
   /// - Warning: After a call to `endCOWMutation` the buffer must not be mutated
   ///   until the next call of `beginCOWMutation`.

stdlib/public/core/SliceBuffer.swift

@@ -329,7 +329,7 @@ internal struct _SliceBuffer<Element>
 
   /// Puts the buffer in an immutable state.
   ///
-  /// - Precondition: The buffer must be mutable.
+  /// - Precondition: The buffer must be mutable or the empty array singleton.
   ///
   /// - Warning: After a call to `endCOWMutation` the buffer must not be mutated
   ///   until the next call of `beginCOWMutation`.