Modify documentation to explain use of parentheses

@solidcell @jacobaweiss @linedotstar 
My apologies, we really meant to open this PR on our forked version of the style guide.

The example you provided of spec usage is probably the one case where we would want to omit the parentheses, because spec examples read as a literal sentence...so thanks for pointing that out! I'm going to modify our own internal PR to include this edge-case. For everything else, we believe it's best to use parentheses as it makes it less likely to introduce bugs with future modifications.