Clarify the difference between attribute and property in Container Docu

[ndricimrr]

<luigi-container viewUrl="/index.html" sandbox-rules='["allow-scripts", "allow-same-origin"]'></luigi-container>
or
myContainer.sandboxRules = ["allow-scripts", "allow-same-origin"]

Are both correct uses.

Discuss of a way to show on the documentation of Container API / Compound Container API that properties and attributes are usually distinguished with a dash.

This seems to be a standard here

Discuss where to put the notice, or if it makes even sense to put any notice.

Whether we put it at the top, of the API page for Container and Compound Container, or for each property separately.

Maybe just a notice
This document outlines the properties provided by the Luigi Container. --- include here a very short statement that mentions the standard for the dash ---