Add some metrics documentation related to #2082 (My Shots button display)
Add some metrics documentation related to #2082 (My Shots button display)

mozilla-services/pageshot,fzzzy/pageshot,mozilla-services/pageshot,fzzzy/pageshot,mozilla-services/pageshot,fzzzy/pageshot,mozilla-services/pageshot,mozilla-services/screenshots,fzzzy/pageshot,mozilla-services/screenshots,mozilla-services/screenshots,mozilla-services/screenshots