Modify documentation to explain use of parentheses

@solidcell @jacobaweiss @linedotstar 
Hrm, you bring up a good point with these edge cases. I think the "rule of lonely parens" is the one case where we might consider omitting them...but we would probably first consider abstracting the object or callback into it's own function before omitting the parens.

I will definitely consider this further. Thanks for your input!