docs: document Tcl procedures

We discussed that we need better documentation on DesyRDL integration into fwk. I started by describing the parameters of addAddressSpace. It should get a short description, too.

I think it makes sense to document the procedures that fwk provides to somebody writing a module/main.tcl in this separate page. If there is no obejction then I will go on with the other procedures that are empty in this draft.