Update YAML reference misleading information about notifications

Problem

There is no mail notification for YAML plans/jobs, but the description in our document reference leads users to believe that we have it.

Notifications

Notifications in Bamboo are triggered by a range of events involving a plan and its jobs, including build completion, build outcomes and comments being posted against build results. You can configure whether notifications are sent for a particular event for each plan and job, and who they are sent to.

Suggested Solution

Inform that notifications are only available for deployments at the moment, otherwise, users might think we didn't document it but it is available.