Change EUIExpression examples and recommendations to avoid 2 clicks

[JasonStoltz]

Is your feature request related to a problem? Please describe. The primary example of EUIExpression shows a configurable "When" value which takes 2 clicks to update.

In actual usage cases in Kibana, this is poor UX. We've heard through a variety of sources (including here) that this is a pain point for users.

This could be accomplished in a single click with a slight variation of that approach, as documented here.

There is nothing preventing users from implementing this today with the current EUIExpression component, we believe this is a result of:

1. Lack of guidance on this issue on the EUIExpression docs page
2. The primary example illustrating the less optimal approach

Describe the solution you'd like I believe this could be solved by:

1. Updating the primary example to use the 1-click approach
2. Offer guidance around when to use a different approach

Describe alternatives you've considered

Desired timeline

Additional context