[fix] correct determination of the IP for the request

For correct determination of the IP to the request the function
botdetection.get_real_ip() is implemented.  This fonction is used in the
ip_limit and link_token method of the botdetection and it is used in the
self_info plugin.

A documentation about the X-Forwarded-For header has been added.

[1] https://github.com/searxng/searxng/pull/2357#issuecomment-1566211059

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

searx/botdetection/__init__.py

@@ -2,11 +2,25 @@
 # lint: pylint
 """.. _botdetection src:
 
-Bot detection methods
----------------------
+X-Forwarded-For
+===============
 
-The methods implemented in this python package are use by the :ref:`limiter src`.
+.. attention::
+
+   A correct setup of the HTTP request headers ``X-Forwarded-For`` and
+   ``X-Real-IP`` is essential to be able to assign a request to an IP correctly:
+
+   - `NGINX RequestHeader`_
+   - `Apache RequestHeader`_
+
+.. _NGINX RequestHeader:
+    https://docs.searxng.org/admin/installation-nginx.html#nginx-s-searxng-site
+.. _Apache RequestHeader:
+    https://docs.searxng.org/admin/installation-apache.html#apache-s-searxng-site
+
+.. autofunction:: searx.botdetection.get_real_ip
 
 """
 
 from ._helpers import dump_request
+from ._helpers import get_real_ip