message: Document `result::bulk_write::modified_count()` throws with mongod 2.4
diff: @@ -57,6 +57,8 @@ class MONGOCXX_API bulk_write {
  ///
  /// @return The number of documents that were modified.
  ///
  + /// @throws with server versions below 2.6 due to the field `nModified` not being returned.
  + ///
  std::int32_t modified_count() const;
  ///
  