Clarify new setRadioPower documentation

There were some questions on how the new IRadio 1.5 setRadioPower
command should be implemented based on vendor feedback. This CL
clarifies this behavior.

Test: manual
Bug: 143683647
Change-Id: If08741f0ea156a5c2656a47e46f7e1f98bf54ec4
Merged-In: If08741f0ea156a5c2656a47e46f7e1f98bf54ec4