Make it clear where the Cue settings for region cues are applied.

Since region cues don't have the following settings:
* vertical
* size
* line
most of the "Apply WebVTT Cue Settings" part is not applicable.
Clarify region scrolling approach.

Closes https://www.w3.org/Bugs/Public/show_bug.cgi?id=22114
Merge pull request #53 from silviapfeiffer/cue_settings

Make it clear where the Cue settings for region cues are applied.