Fix pixel ratio image captions

Clarify actual pixel dimensions and that both are displayed
at same width (200px) to show sharpness difference

Author: adonm

README.md

@@ -58,7 +58,9 @@ Same location, different pixel ratios:
 **Standard (1x)** | **HiDPI (2x)**
 :---:|:---:
 <img src="docs/images/pixelratio-1x.png" width="200"> | <img src="docs/images/pixelratio-2x.png" width="200">
-*400x300 px* | *800x600 px (displayed at 400x300)*
+*400x300 px* | *800x600 px*
+
+Both displayed at 200px width. The 2x version has 4x more pixels for sharper details.
 
 Both images show the exact same geographic area. The 2x version has 4x more pixels for sharper text and details on retina displays.