Fix proxy and mediaproxy description.exs

#2371 (closed), mostly

Trying to clean up this mess and find order in this chaos.

Just because we can allow fine-grained reverse proxy settings for various things doesn't mean we should.

e.g.,

• Pleroma.Upload can force uploads to be proxied by the Pleroma instance, which is useful if you have a hidden upload endpoint on a protected network that only the Pleroma server can access. So if you have this setting enabled, why would you ever want :redirect_on_failure? Clearly it wouldn't work, and if your goal was to conceal your users' IPs from S3/Wasabi/whatever this setting lays a trap.

• Pleroma.Upload, when proxied, can also set a :max_body_length --- but why would you? Shouldn't this be tied directly to :pleroma, :instance, :upload_limit?