gr-rds modules documentation

[David Hagood]

I'm trying to find any documentation on the gr-rds modules - 
specifically, what do they expect to be fed with? It doesn't look like 
they accept IF, nor demodulated FM - it looks like they expect some 
other block between them and the FM discriminator. I've not found any 
docs on the WIKI, nor anything online that describes them.

This is what is shipped in the Ubuntu 20.04 release, in case that matters.