document details on avatar bridging with new endpoint

[kylrth]

From the release notes for v0.7.0, it wasn't clear to me what port I should forward /mautrix-discord/avatar/* to, or if I need to include any forwarding headers. After digging into the code I believe it's just the same port provided to the homeserver for bridge registration, correct?

[kylrth]

Same goes for the direct media functionality: is the media-only Matrix server being served over the same port as everything else? This wasn't clear to me from the docs.

[BatteredBunny]

Its being served on the appservice port, was confusing to me as well.