Link full room deletion in moderation guide

currently doesn't mention anything about actually deleting room contents from your database, so good to add a segment
https://github.com/matrix-org/synapse/blob/develop/docs/admin_api/rooms.md#delete-room-api

Open docs/admin_api/rooms.md from the linked moderation guide and read the existing room-deletion section first. Update the guide so it also covers deleting room contents from the database; the work is done when that procedure is documented alongside the existing deletion guidance.

Written by the indexing model from the issue text.