[doc] adds the missing documentation of the server.method settings.

TL;DR; For all the issues that comes with HTTP POST I recommend instance
maintainers to switch to GET and lock the property in the preferences:

```yaml
server:
  method: GET

preferences:
  lock:
    - method
```

We don't want this in the defaults of the SearXNG distributions for the pros vs
cons listed in this discussion:

- https://github.com/searxng/searxng/pull/3619

docs/admin/settings/settings_server.rst

@@ -14,6 +14,7 @@
        limiter: false
        public_instance: false
        image_proxy: false
+       method: "POST"
        default_http_headers:
          X-Content-Type-Options : nosniff
          X-Download-Options : noopen
@@ -50,8 +51,21 @@
 ``image_proxy`` : ``$SEARXNG_IMAGE_PROXY``
   Allow your instance of SearXNG of being able to proxy images.  Uses memory space.
 
+``method`` : ``GET`` | ``POST``
+  HTTP method.  By defaults ``POST`` is used / The ``POST`` method has the
+  advantage with some WEB browsers that the history is not easy to read, but
+  there are also various disadvantages that sometimes **severely restrict the
+  ease of use for the end user** (e.g. back button to jump back to the previous
+  search page and drag & drop of search term to new tabs do not work as
+  expected .. and several more).  We had a lot of long discussions about the
+  *pros v2 cons*:
+
+  - `set HTTP GET method by default
+    <https://github.com/searxng/searxng/pull/3619>`__
+  - `http methods GET & POST
+    <https://github.com/search?q=repo%3Asearxng%2Fsearxng+label%3A%22http+methods+GET+%26+POST%22>`__
+
 .. _HTTP headers: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers
 
 ``default_http_headers`` :
   Set additional HTTP headers, see `#755 <https://github.com/searx/searx/issues/715>`__
-