Improve Enzyme best practice docs

From #1401, there were some updates to the Enzyme testing docs, but they could be better still.

julienw wrote:
> I'd like some guidance of how to use enzyme; because enzyme is very powerful it's easy to use it in a bad way (eg: manipulating props), and I think we should take great care to test the components from the point of view of the user.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-78)