Add a comment with the format string for --aura-host-window-size

ui/aura/aura_switches.h

Index: ui/aura/aura_switches.h
diff --git a/ui/aura/aura_switches.h b/ui/aura/aura_switches.h
index c6cc0ad24e450889422556f2d777ab5f15e4f251..12cd0265cd18e25b177243d2617e637c86bf2594 100644
--- a/ui/aura/aura_switches.h
+++ b/ui/aura/aura_switches.h
@@ -10,7 +10,13 @@
 namespace switches {
 
 // Please keep alphabetized.
+
+// Sets a window size, optional position, and optional scale factor.
+// "1024x768" creates a window of size 1024x768.
+// "100+200-1024x768" positions the window at 100,200.
+// "1024x768*2" sets the scale factor to 2 for a high DPI display.
 AURA_EXPORT extern const char kAuraHostWindowSize[];
+
 AURA_EXPORT extern const char kAuraHostWindowUseFullscreen[];
 
 }  // namespace switches