Cleanup and add documentation for tags

* Fixed one scenario to work with the default setting
* Fixed one typo in scenario
* Removed unused tags
* Documented the tags currently in use

Change-Id: I14687782857daf79dcd497b934dff99fc45a3997

tests/browser/features/font_selection.feature

@@ -1,4 +1,4 @@
-@reset-preferences-after @login @en.wikipedia.beta.wmflabs.org @commons.wikimedia.beta.wmflabs.org
+@login @reset-preferences-after @en.wikipedia.beta.wmflabs.org @commons.wikimedia.beta.wmflabs.org
 Feature: Font selection
 
   In order to have better using experience,