Document MAS admin APIs as being workerisable (#20209)

### Pull Request Checklist

Replacement for https://github.com/element-hq/synapse/pull/19752 as that
has bit-rotted with https://github.com/element-hq/synapse/pull/19895
being merged.

`/_synapse/mas` is mounted on a worker on matrix.org and can be seen to
be workerisable via
*
https://github.com/element-hq/synapse/blob/v1.160.0/synapse/app/generic_worker.py#L202
*
https://github.com/element-hq/synapse/blob/v1.160.0/synapse/rest/synapse/client/__init__.py#L71
*
https://github.com/element-hq/synapse/blob/v1.160.0/synapse/rest/synapse/mas/__init__.py#L46-L71

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* [x] Pull request is based on the develop branch
* [x] Pull request includes a [changelog
file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog).
The entry should:
- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
  - Use markdown where necessary, mostly for `code blocks`.
  - End with either a period (.) or an exclamation mark (!).
  - Start with a capital letter.
- Feel free to credit yourself, by adding a sentence "Contributed by
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entry.
* [x] [Code
style](https://element-hq.github.io/synapse/latest/code_style.html) is
correct (run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

changelog.d/20209.doc

@@ -0,0 +1 @@
+Document the paths that can be handled on workers with stabilised delegated authentication.

docs/workers.md

@@ -291,6 +291,9 @@ information.
     # Unstable MSC4140 support
     ^/_matrix/client/unstable/org.matrix.msc4140/delayed_events(/.*/restart)?$
 
+    # Stabilised Delegated Authentication support (`matrix_authentication_service.enabled: true`)
+    ^/_synapse/mas/
+
 Additionally, the following REST endpoints can be handled for GET requests:
 
     # Push rules requests
@@ -315,7 +318,7 @@ for the room are in flight:
 
 Additionally, the following endpoints should be included if Synapse is configured
 to use SSO (you only need to include the ones for whichever SSO provider you're
-using):
+using) and delegated authentication isn't enabled:
 
     # for all SSO providers
     ^/_matrix/client/(api/v1|r0|v3|unstable)/login/sso/redirect