:memo: Updates docs with notes for x-data usage

[ekwoka]

This comes up quite often, that someone is stumped by a directive not working, despite copying it from the docs because it doesn't have an x-data parent.

So this adds an extra note in the start here, and a note under the first example that doesn't use x-data for each directive (as needed).

It's a sad day.

[calebporzio]

This is great, sorry for the lag on it. Thanks @ekwoka