Mention how the content repository works over federation

We might want to consider promoting the media repo to it's own API, and maybe consider calling it the Media Repo rather than Content Repo.

Source of information: experience.

specification/server_server_api.rst

@@ -1020,6 +1020,20 @@ the following EDU::
         for the user
 
 
+Content Repository
+------------------
+
+Attachments to events (images, files, etc) are uploaded to a homeserver via the
+Content Repository described in the `Client-Server API`_. When a server wishes
+to serve content originating from a remote server, it needs to ask the remote
+server for the media.
+
+Servers should use the server described in the Matrix Content URI, which has the
+format ``mxc://{ServerName}/{MediaID}``. Servers should use the download endpoint
+described in the `Client-Server API`_, being sure to use the ``allow_remote``
+parameter (set to ``false``).
+
+
 Signing Events
 --------------