[Alerting] Improve wording on "Notify every" form input

[ymao1]

This PR updated the throttle setting on the Create Alert flyout to a dropdown but kept the "Notify every" label. It was brought up that "Notify every" did not make sense with the new wording inside the dropdown.

Discuss if we should change the label, change the dropdown wording, or wait until we move the options to the bottom of the flyout (as shown here) and update the wording at that point.

[elasticmachine]

Pinging @elastic/kibana-alerting-services (Team:Alerting Services)

[arisonl]

cc @gchaps

[gchaps]

@arisonl, @ymao1 How about tweaking both the label and the menu items as follows:

Notify

Only on status change
Actions run when the alert status changes

Every time alert is active
Actions run with every active alert interval

On a custom action interval
Actions run using the interval you set

[ymao1]

That flows much better!