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.
More often than not, it would just be omitted and would default to false. But in the case of a multi-page app, you might not want the user to be able to install the SmartApp until they reach the last page in the flow.

You could also update it based on what the user has set on the page. For example a page could have an input that specified submitOnChange and then toggled the install option based on some custom validation.