The UnknownAction hook was deprecated in 1.19, and has recently started throwing deprecation warnings.
As an extension developer using this hook, I would like to upgrade my extension to use $wgActions. However, I've been unable to find any documentation about how to do this.
The UnknownAction page points to Manual:$wgActions which is where I would probably expect to find the information, but it doesn't tell you very much about how to set up a new action; It just points to Manual:Action.php, which is a bit of a dead-end. There is a not-very-obvious link to the docs for classAction.html but that also doesn't really provide any usage information.
Some documentation needs to be written so that developers can migrate away from this deprecated hook. This needs to happen before the hook can be removed from MediaWiki.