Adding Descriptions to Nodes for Documentation Purposes

[thestillheron]

My team and I are using a deep analysis of our Structr definitions to automatically generate documentation for our API.

I've been wanting to add a "description" property to the Structr nodes for a while, so that we can provide some semantic documentation as well, in a similar way.

This is also handy because we can use it to provide more meaningful error messages if a value in the client's payload isn't right for some reason.