Clarify Child Independence

[JanMiksovsky]

Breaking out this feedback from @arkihillel in #10:

Child Independence — Can you use the component with a wide range of child element types?

This too is unclear. What's considered a wide range? The <span> tag only accepts a very limited set of child elements. Putting a <div> as a child of a <span> will somehow work but it's supposed to be invalid. If the HTML does not always allow child Independence, it shouldn't be considered standard for web components too.

I've expanded the Child Independence topic somewhat. Does that clarify the intent better? The concern here is that devs often create components that only work with a single type of child element, which is generally too limiting.