Update documentation to reflect that run_background_tasks_on is no …

…longer experimental. (#12451)

* Background workers aren't experimental anymore

* Newsfile

Signed-off-by: Olivier Wilkinson (reivilibre) <oliverw@matrix.org>

* Stream writers aren't experimental either

changelog.d/12451.doc

@@ -0,0 +1 @@
+Update documentation to reflect that both the `run_background_tasks_on` option and the options for moving stream writers off of the main process are no longer experimental.

docs/workers.md

@@ -341,9 +341,9 @@ effects of bursts of events from that bridge on events sent by normal users.
 
 #### Stream writers
 
-Additionally, there is *experimental* support for moving writing of specific
-streams (such as events) off of the main process to a particular worker. (This
-is only supported with Redis-based replication.)
+Additionally, the writing of specific streams (such as events) can be moved off
+of the main process to a particular worker.
+(This is only supported with Redis-based replication.)
 
 To enable this, the worker must have a HTTP replication listener configured,
 have a `worker_name` and be listed in the `instance_map` config. The same worker
@@ -420,7 +420,7 @@ the stream writer for the `presence` stream:
 
 #### Background tasks
 
-There is also *experimental* support for moving background tasks to a separate
+There is also support for moving background tasks to a separate
 worker. Background tasks are run periodically or started via replication. Exactly
 which tasks are configured to run depends on your Synapse configuration (e.g. if
 stats is enabled).