Update prefs docs

Our preferences documentation lacks detail, specifically around how to use the various preference elements. A few forum posts have called this out specifically as a source of frustration.

This change set tries to address some of those gaps by providing more documentation on preferences, especially on elements themselves.

@bflorian you may want to look at this, since you have been involved in our preferences documentation.
@smartthingsjstoltz I would appreciate your feedback as well.