Include content around "slotchange" events

[Westbrook]

Particularly, touch on "gotchas" like:

• initial content is not a "change"
• observing is simple meaning childList only
• how assignedNodes/Elements() can flatten through multiple <slot> elements

Outline alternative approaches like Mutation Observers, etc.

[renoirb]

I'd add also examples or use-cases that can make use of "slotchange" (triggering). I've had issues explaining and good documentation would help sharing the knowledge. A use-case can be for transitions of a "value" component. When it's inserted, when the data's there, etc.