Hi Gisele,
binding SOLR to localhost is what you need if everything runs on the same machine: none outside the machine could connect on it: unfortunately, that configuration looks like not a binding but a whitelist which is not the same thing but will work until bugs on the module implementing the whitelist.
I'd try playing w/ https://daniel.hepper.net/blog/2012/04/restricting-ip-addresses-for-solr-with-jetty/ too (http://deeb.me/20160223/bind-solr-5-5-to-localhost).
The Moodle docs covers the installation of the software not the security policies to be applied since they are specific to your environment and usually any OS starts with a basic but nice firewall configuration, enabled by default: do you have the chance to enable a local software firewall opening just HTTP/S and SSH and nothing else?
HTH,
Matteo