[DOCS] Adds Watcher notification account settings

[lcawl]

Per https://www.elastic.co/guide/en/elasticsearch/reference/6.6/breaking-changes-6.6.html#watcher-notifications-account-settings

The following settings have been deprecated and the secure variants should be used instead...
xpack.notification.email.account..smtp.password, instead use xpack.notification.email.account..smtp.secure_password
xpack.notification.hipchat.account..auth_token, instead use xpack.notification.hipchat.account..secure_auth_token
xpack.notification.jira.account..url, instead use xpack.notification.jira.account..secure_url
xpack.notification.jira.account..user, instead use xpack.notification.jira.account..secure_user
xpack.notification.jira.account..password, instead use xpack.notification.jira.account..secure_password
xpack.notification.pagerduty.account..service_api_key, instead use xpack.notification.pagerduty.account..secure_service_api_key
xpack.notification.slack.account..url, instead use xpack.notification.slack.account..secure_url

However, the new settings are missing from the 6.6, 6.7, and 6.8 documentation (https://www.elastic.co/guide/en/elasticsearch/reference/6.6/notification-settings.html)

Related to #35610

[elasticmachine]

Pinging @elastic/es-docs

[elasticmachine]

Pinging @elastic/es-core-features

[lcawl]

These changes can be previewed here: http://elasticsearch_44826.docs-preview.app.elstc.co/guide/en/elasticsearch/reference/6.8/notification-settings.html

[gwbrown]

LGTM, thank you!