Make creation of CPDFSDK_Document clearer

fpdfsdk/include/cpdfsdk_environment.h

Index: fpdfsdk/include/cpdfsdk_environment.h
diff --git a/fpdfsdk/include/cpdfsdk_environment.h b/fpdfsdk/include/cpdfsdk_environment.h
index 338d70f33b9bbfc9d5ffe23a50075e99749d984c..9cd72468747ccf51f7fd9dbb347d24597b0f21d7 100644
--- a/fpdfsdk/include/cpdfsdk_environment.h
+++ b/fpdfsdk/include/cpdfsdk_environment.h
@@ -411,6 +411,9 @@ class CPDFSDK_Environment final {
   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;