I understand your frustration, where did you try to look?
It’s mentioned in the templating section of the documentation and in the view helper reference.
http://neos.readthedocs.io/en/stable/CreatingASite/RenderingCustomMarkup/Templating.html?highlight=uri.resource
http://neos.readthedocs.io/en/stable/References/ViewHelpers/FluidAdaptor.html#f-uri-resource