Update the "expand an ExposedThingInit" steps wrt security definitions

[zolkis]

These steps are sloppily defined wrt security definitions.
https://w3c.github.io/wot-scripting-api/#expand-an-exposedthinginit

[relu91]

Call 24/04/2023:

• SecurityDefinitions are not really well covered throughout the document. We should review them

[JKRhb]

I think we could add this issue to the list in #315 (which we can probably replace with a dedicated GitHub project that deals with security in general at some point). I was also wondering if it is a duplicate of #299 and/or if the two issues could be "merged".