Update ResourceFinder documentation

Add a link to the full ResourceFinder behaviour specification in related docstring documentation. 
For a programmer/user the most natural place to look up the behaviour of the method is the Doxygen documentation
of the method itself, and at the moment there was no link at all in ResourceFinder documentation to the specification. 

cc @lornat75 Not an expert, but I had trouble understand the documentation of ResourceFinder::configure method. Is it still valid or is it outdated?