Update Alert definition for the Alerting plugin

[qreshi]

This is referring to the Key Terms section here:
https://opendistro.github.io/for-elasticsearch-docs/docs/alerting/monitors/

In Alerting, the 'Alert' is the object that represents an event for a particular Trigger. When an Alert is created, subsequent Actions associated with the Trigger are executed one of which can be a notification. The current definition implies that the Alert is the notification which is not correct and can cause confusion around the framework's behavior.

The request is to revisit the Key Terms (mainly Alert) and ensure the distinction between the components is clear.

[keithhc2]

Closed with #483