Add permissions note for manually triggered schedules

What does this MR do?

The documentation is not quite clear under which permissions a manually triggered scheduled pipeline will run. This information is for example needed, if using protected environments where only certain users can execute (on-demand) deployments which are realized via schedules.

This MR adds a note to the docs that manually triggered scheduled pipelines will run with the permissions of the user who triggered them and not with the permissions of the schedule owner (which would be a security issue).

Related issues

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