Add admin api documentation for list media endpoint

Signed-off-by: Travis Ralston <travpc@gmail.com>

docs/admin_api/media_admin_api.md

+# List all media in a room
+
+This API gets a list of known media in a room.
+
+The API is:
+```
+GET /_matrix/client/r0/admin/room/<room_id>/media
+```
+including an `access_token` of a server admin.
+
+It returns a JSON body like the following:
+```
+{
+    "local": [
+        "mxc://localhost/xwvutsrqponmlkjihgfedcba",
+        "mxc://localhost/abcdefghijklmnopqrstuvwx"
+    ],
+    "remote": [
+        "mxc://matrix.org/xwvutsrqponmlkjihgfedcba",
+        "mxc://matrix.org/abcdefghijklmnopqrstuvwx"
+    ]
+}
+```