Update Schema Model

[bleonar5]

TL;DR

Based on the gaps/todos we discover from the validator scoping issue, we want to update the linkML schema model so that it includes some reference to every element of the schema requirements, regardless of whether that reference ends up being explicitly invoked in the validation process.

We also want each element of the schema model to be collated with a human-readable description of either the object or rule at hand, so that we can use this descriptions in the automatic documentation generator.