Improve description of predicate and add real-world examples

[cthoyt]

As someone who might want to use a biolink predicate, I find https://biolink.github.io/biolink-model/#predicates-visualization very difficult to navigate. There are a two things that would help me:

1. Good descriptions for all predicates. A few issue types I found:

   • missing descriptions (e.g., actively_involves)
   • description is the same as the name of the predicate (e.g., longitude)
   • insufficient description(e.g., has_drug)

   Without good descriptions, I don't know what a predicate is actually supposed to be used for.

2. Real world examples of triples that use these (that are fully qualified with terms from well-defined semantic spaces)
   Without these, I find it's often easier to mint my own predicates that may or may not overlap with biolink ones, but at least I can understand the context of.