Update worker docs to update preferred settings for pusher and federation_sender (#14493)

* Fix one typo on line 3700(and apparently do something to other lines, no idea)

* Update config_documentation.md with more information about how federation_senders and pushers settings can be handled.

Specifically, that the instance map style of config does not require the special other variables that enable and disable functionality and that a single worker CAN be added to the map not only just two or more.

* Extra line here for consistency and appearance.

* Add link to sygnal repo.

* Add deprecation notice to workers.md and point to the newer alternative method of defining this functionality.

* Changelog

* Correct version number of Synapse the deprecation is happening in.

* Update quiet deprecation with simple notice and suggestion.