Update explainer to include details about the icon

[strider29]

Added details about the permission icon styling.

[andypaicu]

Looks good, it just needs some more details in the explainer about the icon selector (::permission-icon), perhaps in the "Technical Specification" section?

Something like: ::permission-icon, CSS selector, allows targeting the icon of the element (if there is one), limited styling available (link to the "Annoyance Mitigation" section), the icon itself is selected by the user agent based on the value of the type attribute.