[Value stream analytics] Clarify how date range selector is applied

Summary

There is a date range selector in group-level VSA. The UI and documentation does not clearly explain how selecting a date range affects the stage values and issues/MRs that are included in calculations. This issue highlights a lack of clarity about how date ranges affect VSA data.

Proposal

Per Adam's comment, the date range selector applies to the issue/MR creation date. Stage values are shown for any issues/MRs created within the date range, regardless of whether the issue/MR is still open.

1. Add clarification to the documentation.
2. The UI already has a tooltip stating the maximum period that can be specified. Add the following text to the beginning of the tooltip. "Data is shown for workflow items created during the selected date range."

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