Editorial and definition changes based on Med's review of -03

[renghardt]

This should cover everything discussed in #23 except the use cases section, which we tackle in another issue. I added the definitions we promised to add while limiting our scope to definitions, i.e., not trying to go into how paths and path properties can be discovered. That's also why I cut some details for the "Transport Protocols available" property: Sure, a host can cache this because it's visible locally, but also based on probing as Med correctly pointed out, but let's try to avoid going into these different possibilities here.

[cyrill-k]

I think this looks fine now, so from my side, we can merge it