Clarify role of feedbackbox

Adds a small description to the feedback box as solution of  [hhucn/adhocracy.hhu_theme/Issue#410](https://github.com/hhucn/adhocracy.hhu_theme/issues/410)

The .po files directly come from "tx pull". But somehow some translations have changed that I didn't edit:

-msgid "Intro"
-msgstr "Intro"

-msgid "Overview"
-msgstr "Overview"
+msgid "IntroOverview"
+msgstr "IntroOverview"