Clarify one comment about not using think time

[MattDodsonEnglish]

The "Think time" doc says that users might not use want think time when:

Your load generator can run your test script without crossing the 80% CPU utilization mark.

Why is this the case? It's not so obvious to me how this pertains to the decision to use sleep.

[nicolevanderhoeven]

Good point. I tried to explain it in the previous section, but it could use some work.

How about this?