Add some more discussion of asynchrony.
Add some more discussion of asynchrony.

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1472883003 .

iglherme/Guttiest,kevmoo/test,grouma/test,iglherme/Guttiest,grouma/test,kevmoo/test,kevmoo/test,grouma/test,iglherme/Guttiest