[CI:DOCS] Add information about Type=oneshot units in podman-systemd.unit.5.md (fixes #22233)

[marinmo]

Add information about Type=oneshot units as suggested in https://github.com/containers/podman/issues/22233

Does this PR introduce a user-facing change?

None

[openshift-ci[bot]]

Adding the "do-not-merge/release-note-label-needed" label because no release-note block was detected, please follow our release note process to remove it.

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[openshift-ci[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: marinmo, ygalblum

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[rhatdan]

/lgtm