Document option to use the MediaProxyWarmingPolicy and nginx setup to cache remote media 100%
Pleroma doesn't copy remote media, and can either hotlink or proxy it depending on the instance's config. While this is good for 99% of users, some people have expressed interest in having their instance act as a long-term cache, so media from other instances is not broken when the instance has downtime or shut down.
Pleroma can cache images through the media proxy and nginx. Some people want to cache all remote media, which can be achieved by an unlimited / non-expiring nginx cache and the MediaProxyWarmingPolicy. This should be documented so people know how to it.