update Duration and cpuTime descriptions per dicussion

Updating description for 'duration' and 'cpuTime' parameters for issue #3.

This clarifies a few points: 
1) duration is measured as per requestAnimationFrame's definition of "start time" until the next VSYNC boundary this frame can make. This means duration will be approximately 16, 32, etc for ~60 FPS, depending on if you made or missed a particular deadline. It is a binary signal showing that you are missing deadlines.

2) 'cpuTime' captures wall clock EXCLUDING idle time. It is meant to show how much headroom you have when you make a deadline, or how much you missed by, when you miss one (or more.)

3) cpuTime is not an OS-level CPU time, it continues counting if the thread is descheduled and is a MAX() not a SUM() of all concurrent threads processing (if there are more than one thread). This is intentional because as a web developer, you have some amount of resources to use and are trying to use them to achieve a desired effect (smoothly!) It is the kernel and the user agent's job to manage system resources, and distribute time fairly. 

Please let me know if the wording is clear, and if this makes sense in a vendor-agnostic way.
Thanks!
- what's the reason for "possible" in "possible v-sync" boundary? can it be omitted? 
- what's the reason to include "or, startTime of next frame", wouldn't the first v-sync clause cover all the necessary cases?
- pedantic... I'd simplify the last bit as "duration captures the wall-clock duration of the frame"?