Document that the new get single delayed event endpoint is workerisable (#20210)

### Pull Request Checklist

Missed off of https://github.com/element-hq/synapse/pull/19926/changes
IMO. My reading of
https://github.com/element-hq/synapse/blob/v1.161.0rc1/synapse/rest/client/delayed_events.py
is that the new endpoint (pattern
`r"/org\.matrix\.msc4140/delayed_events/(?P<delay_id>[^/]+)$"` is
workerisable) given how `register_servlets` flows. However given
`UpdateDelayedEventServlet` covers the same pattern but for `POST`
requests, this should go in the `GET` only part of the documentation.

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changelog.d/20210.doc

@@ -0,0 +1 @@
+Document the endpoint for getting a single delayed event is workerisable.

docs/workers.md

@@ -309,6 +309,9 @@ Additionally, the following REST endpoints can be handled for GET requests:
     # Admin API requests
     ^/_synapse/admin/v2/users/[^/]+$
 
+    # Unstable MSC4140 support
+    ^/_matrix/client/unstable/org.matrix.msc4140/delayed_events/[^/]+$
+
 Pagination requests can also be handled, but all requests for a given
 room must be routed to the same instance. Additionally, care must be taken to
 ensure that the purge history admin API is not used while pagination requests