Improve pipeline status notifications

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As discussed in these issues, https://gitlab.com/gitlab-org/gitlab-ce/issues/25028#note_26072482 and https://gitlab.com/gitlab-org/gitlab-ce/issues/24845 customers are having difficulties in understanding the default behavior of the notification from our documentation.

For example: ZD: https://gitlab.zendesk.com/agent/tickets/79274

@godfat your thoughts?

cc// @axil

Edit: Previous Title: Improve documentation for pipeline status notifications. Updated due to #18656 (comment 215013298)