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.
You should also include the *install* option. Without install:true the SmartApp cannot be installed and therefore will never execute. Likewise, without uninstall:true, the SmartApp can never be uninstalled.