Where can i find a brief description of HTTP Query smart service, other than

All the information available for this plug-in is listed in the documentation which is actually the link you describe in your post. There's no other place where an out of the box feature is documented other than in the documentation.

As explained in that link: "HTTP Query Smart Service Appian 7.7
The HTTP Query smart service allows you to make HTTP requests, suitable for consuming the REST API of an external system. " but even if you have a web service that requires a SOAP envelope you can still invoke it with this plug-in by building your own envelope (i.e. text) and passing it as the body of the request via POST.