XFA: add underlying document type comment

fpdfsdk/include/fsdk_mgr.h

 // Copyright 2014 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_FSDK_MGR_H_
 #define FPDFSDK_INCLUDE_FSDK_MGR_H_
 
 #include <map>
@@ skipping 461 matching lines @@
   nonstd::unique_ptr<IFX_SystemHandler> m_pSysHandler;
 };
 
 class CPDFSDK_Document {
  public:
   CPDFSDK_Document(UnderlyingDocumentType* pDoc, CPDFDoc_Environment* pEnv);
   ~CPDFSDK_Document();
 
   CPDFSDK_InterForm* GetInterForm();
 
+  // Gets the document object for the next layer down; for master this is
+  // a CPDF_Document, but for XFA it is a CPDFXFA_Document.
   UnderlyingDocumentType* GetUnderlyingDocument() const {
     return GetXFADocument();
   }
+
+  // Gets the CPDF_Document, either directly in master, or from the
+  // CPDFXFA_Document for XFA.
   CPDF_Document* GetPDFDocument() const {
     return m_pDoc ? m_pDoc->GetPDFDoc() : nullptr;
   }
+
+  // Gets the XFA document directly (XFA-only).
   CPDFXFA_Document* GetXFADocument() const { return m_pDoc; }
 
   int GetPageViewCount() const { return m_pageMap.size(); }
   CPDFSDK_PageView* GetPageView(UnderlyingPageType* pPDFXFAPage,
                                 FX_BOOL ReNew = TRUE);
   CPDFSDK_PageView* GetPageView(int nIndex);
   CPDFSDK_PageView* GetCurrentView();
   void RemovePageView(UnderlyingPageType* pPDFPage);
   void UpdateAllViews(CPDFSDK_PageView* pSender, CPDFSDK_Annot* pAnnot);
 
@@ skipping 184 matching lines @@
     }
 
     if (nStartPos < m)
       QuickSort(nStartPos, m, bAscend, pCompare);
     if (nStopPos > m)
       QuickSort(m, nStopPos, bAscend, pCompare);
   }
 };
 
 #endif  // FPDFSDK_INCLUDE_FSDK_MGR_H_