Make creation of CPDFSDK_Document clearer

fpdfsdk/include/cpdfsdk_environment.h

 // Copyright 2016 PDFium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Original code copyright 2014 Foxit Software Inc. http://www.foxitsoftware.com
 
 #ifndef FPDFSDK_INCLUDE_CPDFSDK_ENVIRONMENT_H_
 #define FPDFSDK_INCLUDE_CPDFSDK_ENVIRONMENT_H_
 
 #include <memory>
@@ skipping 393 matching lines @@
   CFFL_InteractiveFormFiller* GetInteractiveFormFiller();
   CPDFSDK_AnnotHandlerMgr* GetAnnotHandlerMgr();  // Creates if not present.
   IJS_Runtime* GetJSRuntime();                    // Creates if not present.
   CPDFSDK_ActionHandler* GetActionHander();       // Creates if not present.
 
  private:
   std::unique_ptr<CPDFSDK_AnnotHandlerMgr> m_pAnnotHandlerMgr;
   std::unique_ptr<CPDFSDK_ActionHandler> m_pActionHandler;
   std::unique_ptr<IJS_Runtime> m_pJSRuntime;
   FPDF_FORMFILLINFO* const m_pInfo;
+  // Ownership of |m_pSDKDoc| depends on if this is XFA. If we're in XFA then
+  // the object is owned by the CPDFXFA_Document. In non-xfa then we own
+  // the pointer.
   CPDFSDK_Document* m_pSDKDoc;
   UnderlyingDocumentType* const m_pUnderlyingDoc;
   std::unique_ptr<CFFL_InteractiveFormFiller> m_pFormFiller;
   std::unique_ptr<CFX_SystemHandler> m_pSysHandler;
 };
 
 #endif  // FPDFSDK_INCLUDE_CPDFSDK_ENVIRONMENT_H_