[docs] document settings.yml: search.languages

Requested-by: @dalf https://github.com/searxng/searxng/pull/996#discussion_r830858139
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

searx/settings.yml

@@ -21,6 +21,15 @@ search:
   # Default search language - leave blank to detect from browser information or
   # use codes from 'languages.py'
   default_lang: ""
+  # Available languages
+  # languages:
+  #   - all
+  #   - en
+  #   - en-US
+  #   - de
+  #   - it-IT
+  #   - fr
+  #   - fr-BE
   # ban time in seconds after engine errors
   ban_time_on_fail: 5
   # max ban time in seconds after engine errors