Threading doc improvements #288

Steven Vukelich (@vukelich) has two suggestions:

• Quick dictionary for some of the major terms. "Join"/"Joinable" was something I think seemed to cause confusion, for example.
• A diagram that shows the relationship between the JTF, a joinable collection, a joinable task, regular Tasks and TaskCompletionSources, and how these prevent or cause a hang by violating rule Fix hang in AsyncLazy<T> when value factory synchronously blocks for UI thread #3.