Add some basic admin API docs

docs/admin_api/README.rst

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+Admin APIs
+==========
+
+This directory includes documentation for the various synapse specific admin
+APIs available.
+
+Only users that are server admins can use these APIs. A user can be marked as a
+server admin by updating the database directly, e.g.:
+
+``UPDATE users SET admin = 1 WHERE name = '@foo:bar.com'``
+
+Restarting may be required for the changes to register.

docs/admin_api/purge_history_api.rst

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+Purge History API
+=================
+
+The purge history API allows server admins to purge historic events from their
+database, reclaiming disk space.
+
+Depending on the amount of history being purged a call to the API may take
+several minutes or longer. During this period users will not be able to
+paginate further back in the room from the point being purged from.
+
+The API is simply:
+
+``POST /_matrix/client/r0/admin/purge_history/<room_id>/<event_id>``
+
+including an ``access_token`` of a server admin.

docs/admin_api/purge_remote_media.rst

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+Purge Remote Media API
+======================
+
+The purge remote media API allows server admins to purge old cached remote
+media. 
+
+The API is::
+
+    POST /_matrix/client/r0/admin/purge_history/
+
+    {
+        "before_ts": <unix_timestamp_in_ms>
+    }
+
+Which will remove all cached media that was last accessed before
+``<unix_timestamp_in_ms>``.
+
+If the user re-requests purged remote media, synapse will re-request the media
+from the originating server.