Remove documentation of legacy `frontend_proxy` worker app (#13645)

This has been the same as a generic_worker since #6964, so let's get rid of it.

Fixes #3717

changelog.d/13645.doc

+Remove documentation of legacy `frontend_proxy` worker app.

docs/systemd-with-workers/workers/generic_worker.yaml

@@ -5,6 +5,8 @@ worker_name: generic_worker1
 worker_replication_host: 127.0.0.1
 worker_replication_http_port: 9093
 
+worker_main_http_uri: http://localhost:8008/
+
 worker_listeners:
   - type: http
     port: 8083

docs/workers.md

@@ -117,7 +117,7 @@ redis:
     enabled: true
 ```
 
-See the sample config for the full documentation of each option.
+See the [configuration manual](usage/configuration/config_documentation.html) for the full documentation of each option.
 
 Under **no circumstances** should the replication listener be exposed to the
 public internet; replication traffic is:
@@ -128,15 +128,15 @@ public internet; replication traffic is:
 
 ### Worker configuration
 
-In the config file for each worker, you must specify the type of worker
-application (`worker_app`), and you should specify a unique name for the worker
-(`worker_name`). The currently available worker applications are listed below.
-You must also specify the HTTP replication endpoint that it should talk to on
-the main synapse process.  `worker_replication_host` should specify the host of
-the main synapse and `worker_replication_http_port` should point to the HTTP
-replication port. If the worker will handle HTTP requests then the
-`worker_listeners` option should be set with a `http` listener, in the same way
-as the `listeners` option in the shared config.
+In the config file for each worker, you must specify:
+ * The type of worker (`worker_app`). The currently available worker applications are listed below.
+ * A unique name for the worker (`worker_name`).
+ * The HTTP replication endpoint that it should talk to on the main synapse process
+   (`worker_replication_host` and `worker_replication_http_port`)
+ * If handling HTTP requests, a `worker_listeners` option with an `http`
+   listener, in the same way as the `listeners` option in the shared config.
+ * If handling the `^/_matrix/client/v3/keys/upload` endpoint, the HTTP URI for
+   the main process (`worker_main_http_uri`).
 
 For example:
 
@@ -220,10 +220,12 @@ information.
     ^/_matrix/client/(api/v1|r0|v3|unstable)/search$
 
     # Encryption requests
+    # Note that ^/_matrix/client/(r0|v3|unstable)/keys/upload/ requires `worker_main_http_uri`
     ^/_matrix/client/(r0|v3|unstable)/keys/query$
     ^/_matrix/client/(r0|v3|unstable)/keys/changes$
     ^/_matrix/client/(r0|v3|unstable)/keys/claim$
     ^/_matrix/client/(r0|v3|unstable)/room_keys/
+    ^/_matrix/client/(r0|v3|unstable)/keys/upload/
 
     # Registration/login requests
     ^/_matrix/client/(api/v1|r0|v3|unstable)/login$
@@ -584,39 +586,16 @@ handle it, and are online.
 If `update_user_directory` is set to `false`, and this worker is not running,
 the above endpoint may give outdated results.
 
-### `synapse.app.frontend_proxy`
-
-Proxies some frequently-requested client endpoints to add caching and remove
-load from the main synapse. It can handle REST endpoints matching the following
-regular expressions:
-
-    ^/_matrix/client/(r0|v3|unstable)/keys/upload
-
-If `use_presence` is False in the homeserver config, it can also handle REST
-endpoints matching the following regular expressions:
-
-    ^/_matrix/client/(api/v1|r0|v3|unstable)/presence/[^/]+/status
-
-This "stub" presence handler will pass through `GET` request but make the
-`PUT` effectively a no-op.
-
-It will proxy any requests it cannot handle to the main synapse instance. It
-must therefore be configured with the location of the main instance, via
-the `worker_main_http_uri` setting in the `frontend_proxy` worker configuration
-file. For example:
-
-```yaml
-worker_main_http_uri: http://127.0.0.1:8008
-```
-
 ### Historical apps
 
-*Note:* Historically there used to be more apps, however they have been
-amalgamated into a single `synapse.app.generic_worker` app. The remaining apps
-are ones that do specific processing unrelated to requests, e.g. the `pusher`
-that handles sending out push notifications for new events. The intention is for
-all these to be folded into the `generic_worker` app and to use config to define
-which processes handle the various proccessing such as push notifications.
+The following used to be separate worker application types, but are now
+equivalent to `synapse.app.generic_worker`:
+
+ * `synapse.app.client_reader`
+ * `synapse.app.event_creator`
+ * `synapse.app.federation_reader`
+ * `synapse.app.frontend_proxy`
+ * `synapse.app.synchrotron`
 
 
 ## Migration from old config