Document NetworkPolicy in ServiceDiscovery Manual

https://access.redhat.com/documentation/en-us/red_hat_3scale_api_management/2.4/html-single/service_discovery/index#making-service-discoverable

Customers on clusters that have the ovs-networkpolicy plugin may require NetworkPolicy objects created within their app project that allow traffic to/from the openshift-3scale project. This affects RHMI and OSD customers and may need documentation links from their docs as well.

https://access.redhat.com/documentation/en-us/red_hat_managed_integration/1/html-single/

https://docs.openshift.com/dedicated/3/admin_guide/managing_networking.html#admin-guide-networking-networkpolicy

Without the NetworkPolicy object, users will get an "execution expired" error as it times out attempting to connect to the service.