Improve description of relationship between DCAT and PROF

[nicholascar]

Motivated by the suggestion here w3c/dxwg#808 (comment) and the points here w3c/dxwg#808 (comment) which are:

I have trouble with this characterization in DCAT:

"The main subject that differentiates PROF from DCAT is that PROF specifically addresses the notion of conformance – of profiles to specifications or other profiles – about which DCAT is silent. DCAT does address the notion of instance data conforming to an "established standard" (see it's suggested use of the dct:conformsTo property and here – instance data conformance to specifications – DCAT and PROF offer compatible instructions. "

My problem is that it's kind of saying "the apples don't look like oranges". That DCAT does not address conformance between distributions is logically irrelevant to DCAT. So I wouldn't put this in a description of differences because I don't think it makes sense. DCAT and PROF have distinctly different functions. It WOULD be interesting to explain more about what it means that PROF borrows those structures. Right now there is only one sentence, and I'm not sure that it gives much information to someone who doesn't know DCAT.

"PROF borrows its main structures from DCAT in that PROF's prof:Profile & prof:ResourceDescriptor classes parallel DCAT's dcat:Dataset & dcat:Distribution classes. "

Something perhaps about the profile/dataset classes represent abstract works, and that these works are expressed in distributions which embody the meaning of the abstract work. (OMG, that's even worse! Sorry, but maybe you can see what I mean.)