added a note that the green translation comes from the phrasebook

src/ui/android/assets/help_content.html

@@ -23,6 +23,10 @@ and gives control on quality. It indicates confidence with colours:
                  Builds the translation word by word.
                 </li>
 </ul>
+The green translations come from a tourist phrasebook, which allows
+you to translate things like "hello" and "how far is the airport from
+the hotel". But this part can be easily adapted to other uses, such as
+technical language.
 </p>
 
 <p>