Add clearer doc for JSON schema's default keyword

[mothslaw]

Is your feature request related to a problem? Please describe.
A common misconception is that the default keyword in JSON schemas means "If the user does not supply a value for this field, then the default value will be auto-substituted". But, that's not what it means. In fact, no auto-substitution is done by the engine or anyone else. Although, our UI does, in some cases, use the {{default}} to pre-fill widgets.

Describe the solution you'd like
In the doc, we should warn plugin developers of this potential misunderstanding.