[Docs] Document the re-introduced 'ssl.trust' email account setting for watchers

[hoigau]

With reference to the PR#31684, the 'ssl.trust' setting is introduced from v6.3.2 to mark the remote entity as trusted in order to support SSL communication with the destination for watches.

The current doc includes only the ssl setup for HTTP:

https://www.elastic.co/guide/en/elasticsearch/reference/6.3/notification-settings.html#ssl-notification-settings

Need to have the setting documented for watcher setup to work with remote destination requiring SSL support at other non-HTTP notifications.

[elasticmachine]

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