Provide example permission policy control via JS Element.setAttribute()

[michaelwasserman]

It may be valuable for the spec and/or explainer to show some example code using Element.setAttribute() on an iframe in JS to allow some permission, beyond specifying the 'allow' attribute in html. e.g. document.getElementById('iframe').setAttribute('allow', 'geolocation');

I wonder if's also helpful to clarify how that is handled after the document is loaded, since, for example section 6.2 says:

When not empty, the "allow" attribute will result in adding an allowlist for each recognized feature to the iframe element’s nested browsing context's container policy, when it is constructed.
What is expected to happen when the attribute is modified after construction?