Update worker docs with recent enhancements (#7969)

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+### Using synctl with workers
+
+If you want to use `synctl` to manage your synapse processes, you will need to
+create an an additional configuration file for the main synapse process. That
+configuration should look like this:
+
+```yaml
+worker_app: synapse.app.homeserver
+```
+
+Additionally, each worker app must be configured with the name of a "pid file",
+to which it will write its process ID when it starts. For example, for a
+synchrotron, you might write:
+
+```yaml
+worker_pid_file: /home/matrix/synapse/worker1.pid
+```
+
+Finally, to actually run your worker-based synapse, you must pass synctl the `-a`
+commandline option to tell it to operate on all the worker configurations found
+in the given directory, e.g.:
+
+    synctl -a $CONFIG/workers start
+
+Currently one should always restart all workers when restarting or upgrading
+synapse, unless you explicitly know it's safe not to.  For instance, restarting
+synapse without restarting all the synchrotrons may result in broken typing
+notifications.
+
+To manipulate a specific worker, you pass the -w option to synctl:
+
+    synctl -w $CONFIG/workers/worker1.yaml restart