Update GPU Pixel Wrangler Docs for pixel_skia_gold_tests

The section on pixel tests lists an output link format. However for the
pixel_skia_gold_tests the link format is different.

This updates the docs to include a note about the differences in links, with an
example.

Change-Id: I444c5391a3c05561b4177a76a7d3db70d4f03361
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1638548
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jonathan Ross <jonross@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665247}

docs/gpu/pixel_wrangling.md

@@ -204,6 +204,8 @@ shift, and a calendar appointment.
         the "pixel" step.
     1.  The output will contain a link of the form
         <http://chromium-browser-gpu-tests.commondatastorage.googleapis.com/view_test_results.html?242523_Linux_Release_Intel__telemetry>
+        * For pixel_skia_gold_tests the output link will be of the form
+        <https://chrome-gpu-gold.skia.org/detail?test=Pixel_BackgroundImage&digest=13c038c8d426720ae575f4cb9f0bf8da>
     1.  Visit the link to see whether the generated or reference images look
         incorrect.
     1.  All of the reference images for all of the bots are stored in cloud