Explain that window.find() is not relevant

[domenic]

Given that it's a topic of recent discussion (whatwg/html#3539), some folks might wonder what the connection is. Add an appendix at the bottom of the main explainer (e.g., "Related technologies") that explains what window.find() does, and how it doesn't help with our use cases at all.

[rakina]

I added https://github.com/rakina/find-in-page-api#windowfind, should we also mention FindText API?

[domenic]

Yeah, that'd probably be a good idea. The relationship there is a bit more complicated, but probably worth talking about.