[mod] document server:public_instance & remove it out of the botdetection

- the option server:public_instance lacks some documentation
- the processing of this option belongs in the limiter and not
  in botdetection module

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

docs/admin/settings/settings_server.rst

@@ -12,6 +12,7 @@
        bind_address: "127.0.0.1"
        secret_key: "ultrasecretkey"           # change this!
        limiter: false
+       public_instance: false
        image_proxy: false
        default_http_headers:
          X-Content-Type-Options : nosniff
@@ -20,7 +21,6 @@
          X-Robots-Tag : noindex, nofollow
          Referrer-Policy : no-referrer
 
-
 ``base_url`` : ``$SEARXNG_URL`` :ref:`buildenv <make buildenv>`
   The base URL where SearXNG is deployed.  Used to create correct inbound links.
   If you change the value, don't forget to rebuild instance's environment
@@ -40,6 +40,16 @@
   Rate limit the number of request on the instance, block some bots.  The
   :ref:`limiter` requires a :ref:`settings redis` database.
 
+.. _public_instance:
+
+``public_instance`` :
+
+  Setting that allows to enable features specifically for public instances (not
+  needed for local usage).  By set to ``true`` the following features are
+  activated:
+
+  - :py:obj:`searx.botdetection.link_token` in the :ref:`limiter`
+
 .. _image_proxy:
 
 ``image_proxy`` :