Update geoip.asciidoc (elastic#105908)

The GeoIP endpoint does not use the xpack http client. The GeoIP downloader uses the JDKs builtin cacerts.

If customer is using custom https endpoint they need to provide the cacert in the jdk, whether our jdk bundled in or their jdk. Otherwise they will see something like
```
...PKiX path building failed: sun.security.provier.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target...
```

docs/reference/ingest/processors/geoip.asciidoc

@@ -435,6 +435,8 @@ each node's <<es-tmpdir,temporary directory>> at `$ES_TMPDIR/geoip-databases/<no
 Note that {es} will make a GET request to `${ingest.geoip.downloader.endpoint}?elastic_geoip_service_tos=agree`,
 expecting the list of metadata about databases typically found in `overview.json`.
 
+The GeoIP downloader uses the JDK's builtin cacerts. If you're using a custom endpoint, add the custom https endpoint cacert(s) to the JDK's truststore.
+
 [[ingest-geoip-downloader-poll-interval]]
 `ingest.geoip.downloader.poll.interval`::
 (<<dynamic-cluster-setting,Dynamic>>, <<time-units,time value>>)