Add documentation of Custom URL requests

Adds basic documentation to the API. Though the API is still private
we would like people to experiment with it, which is easy if it is
documented.

Change-Id: I63985dfc16dc91270d95edf6fc306f9c71a10f29
Reviewed-by: Andras Becsi <andras.becsi@theqtcompany.com>
Reviewed-by: Florian Bruhin <qt-project.org@the-compiler.org>