Clarify last item of region processing model in regions

[gkatsev]

When implementing region support in vttjs (videojs/vtt.js#50), I was a bit unsure what section 7.1 step 14->3->10 referred to (https://w3c.github.io/webvtt/#processing-model). I think I it refers to collecting the boxes and adjusting the top to account for new cue boxes in the region container as new items are added when scroll is set to up but would be good to update the text so that it's absolutely clear what the meaning is.