Add direct references to terms/concepts

[aragilar]

Given TAP builds on a large number of existing recommendations, it can get quite confusing to work out where terms are defined/what values can be used, especially the values for TAP_SCHEMA.columns. UCDs have a standalone recommendation (and so are somewhat easier to understand), though they'd benefit from having a direct reference to the recommendation, but utypes and xtypes seem to have no easy entry point (especially for astronomers who might be trying to provide metadata for their data, and not implement a TAP system). TAP 1.0 did have a bit more clarity in the tables around the columns purposes (which seems to have disappeared from TAP 1.1), but having direct references to the relevant standards would be make the structure of the system clearer.