[docs] document the missing server:limiter setting

BTW: fix some minor typos in docs/admin/engines/settings.rst

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

docs/admin/engines/settings.rst

@@ -168,6 +168,7 @@ Global Settings
        port: 8888
        bind_address: "127.0.0.1"      # address to listen on
        secret_key: "ultrasecretkey"   # change this!
+       limiter: false
        image_proxy: false             # proxying image results through SearXNG
        default_http_headers:
          X-Content-Type-Options : nosniff
@@ -194,6 +195,10 @@ Global Settings
 ``secret_key`` : ``$SEARXNG_SECRET``
   Used for cryptography purpose.
 
+``limiter`` :
+  Rate limit the number of request on the instance, block some bots.  The
+  :ref:`limiter plugin` requires a :ref:`settings redis` database.
+
 ``image_proxy`` :
   Allow your instance of SearXNG of being able to proxy images.  Uses memory space.