Improve Concept Link Descriptions

The descriptions for the links in _concepts/**/links.json_ are based on the markdown arbitrary reference text for reference-style links. For the most part, these work well but some are a bit cryptic.

For instance, the `time` concept contains links described as _skeet-time-of-day_ and _wiki-utc_. These are probably better as "time-of-day per Jon Skeet" and "Wikipedia on UTC".

I think brevity is an aspiration for the link descriptions but I think it's worth mentioning the source if it's Stack Overflow or Wikipedia or a well-known authority and their view is adding color to our description of the concept which is covered by more generic links.