[CAM-13480] In the process definition history view, the time restriction label cleary describes its semantics

In the process definition history view, there is a feature to restrict the time span for which historic activity instances are displayed in the diagram. This is based on the time-based filtering criteria of the following request: https://docs.camunda.org/manual/7.15/reference/rest/history/process-definition/get-historic-activity-statistics/.
It filters by the start date of the corresponding process instance, and not by the start date of the activity instances in the result. The UI should make this distinction clear.

Example:
- The time period is "Today"
- A process instance was started three days ago
- Four activity instances of the process instance were executed today
- The activity instances are not counted in the displayed statistics