Add PDF reference to CFX_Matrix comment

Change-Id: I801f3cf11a27f91fa3e75518143f953050b2f607
Reviewed-on: https://pdfium-review.googlesource.com/4650
Reviewed-by: dsinclair <dsinclair@chromium.org>
Commit-Queue: dsinclair <dsinclair@chromium.org>
diff --git a/core/fxcrt/fx_coordinates.h b/core/fxcrt/fx_coordinates.h
index cbae45b..fc51c45 100644
--- a/core/fxcrt/fx_coordinates.h
+++ b/core/fxcrt/fx_coordinates.h
@@ -570,6 +570,7 @@
 // | a  b  0 |
 // | c  d  0 |
 // | e  f  1 |
+// See PDF spec 1.7 Section 4.2.3.
 //
 class CFX_Matrix {
  public: