Expose /_synapse/* APIs via matrix-synapse-reverse-proxy-companion

This also updates validation tasks and documentation, pointing to
variables in the matrix-synapse role which don't currently exist yet
(e.g. `matrix_synapse_container_labels_client_synapse_admin_api_enabled`).

These variables will be added soon, as Traefik labels are added to the
`matrix-synapse` role. At that point, the `matrix-synapse-reverse-proxy-companion` role
will be updated to also use them.

docs/maintenance-synapse.md

@@ -16,9 +16,9 @@ Table of contents:
 
 You can use the **[Purge History API](https://github.com/matrix-org/synapse/blob/master/docs/admin_api/purge_history_api.md)** to delete old messages on a per-room basis. **This is destructive** (especially for non-federated rooms), because it means **people will no longer have access to history past a certain point**.
 
-To make use of this API, **you'll need an admin access token** first. Refer to the documentation on [how to obtain an access token](obtaining-access-tokens.md).
+To make use of this Synapse Admin API, **you'll need an admin access token** first. Refer to the documentation on [how to obtain an access token](obtaining-access-tokens.md).
 
-Synapse's Admin API is not exposed to the internet by default. To expose it you will need to add `matrix_nginx_proxy_proxy_matrix_client_api_forwarded_location_synapse_admin_api_enabled: true` to your `vars.yml` file.
+Synapse's Admin API is not exposed to the internet by default, following [official Synapse reverse-proxying recommendations](https://github.com/matrix-org/synapse/blob/master/docs/reverse_proxy.md#synapse-administration-endpoints). To expose it you will need to add `matrix_synapse_container_labels_client_synapse_admin_api_enabled: true` to your `vars.yml` file.
 
 Follow the [Purge History API](https://github.com/matrix-org/synapse/blob/master/docs/admin_api/purge_history_api.md) documentation page for the actual purging instructions.