Define behaviors of FPDF_RenderPageBitmap_Retail and FPDF_FFLDraw.

public/fpdfview.h

Index: public/fpdfview.h
diff --git a/public/fpdfview.h b/public/fpdfview.h
index cbb577b2fd476c93b05dc9960112722de2dfcc14..499124d6e8d4c354ad82b763e9c83feca7eb32cc 100644
--- a/public/fpdfview.h
+++ b/public/fpdfview.h
@@ -595,7 +595,10 @@ DLLEXPORT void STDCALL FPDF_RenderPage(HDC dc,
 //                            2 (rotated 180 degrees)
 //                            3 (rotated 90 degrees counter-clockwise)
 //          flags       -   0 for normal display, or combination of flags
-//                          defined above.
+//                          defined above. With FPDF_ANNOT flag, it renders all
+//                          annotations that does not require user-interaction,
+//                          which are all annotations except widget and popup
+//                          annotations.
 // Return value:
 //          None.
 DLLEXPORT void STDCALL FPDF_RenderPageBitmap(FPDF_BITMAP bitmap,