Merge #2062

2062: kernel: sched: add and update comments r=hudson-ayers a=bradjc

Slight tweak to how process time used is calculated.

### Pull Request Overview

This pull request mostly just adds comments to sched. I think this code is has been looked at a fair bit recently, and its worth clearly explaining what is going on for anyone interested in reading through the main kernel loop.

I did change how time used is calculated, as the comments didn't quite seem to match the code.


### Testing Strategy

travis? Mostly just comments.


### TODO or Help Wanted

n/a


### Documentation Updated

- [x] Updated the relevant files in `/docs`, or no updates are required.

### Formatting

- [x] Ran `make prepush`.


Co-authored-by: Brad Campbell <bradjc5@gmail.com>
Co-authored-by: Pat Pannuto <pat.pannuto@gmail.com>