Levels documentation

[joycecheng]

I was not able to figure out the meaning of the levels argument to many of the terms from the existing documentation. Left them as a TODO for now. Alternatively, levels explain is templated. If its function is already well understood, I can replace the TODOs with the template.

[krivit]

We might want to make a levels template that takes a template variable, which lets the writer specify what the argument does, then inserts the standard attribute UI links.

[joycecheng]

Converted them into templates but left the explanation as TODO