[VSA] Explain definition of "lead time" (ordinary vs DORA)

Problem to solve

Under #300472 (closed) VSA started displaying DORA "Lead Time For Changes" to Premium users, while continuing to display ordinary "Lead Time" to Free users. For many (most?) users, the difference between these metrics are not clear and should be explained.

Proposal

In VSA, add a popover in the existing popover for the "lead time" metric, display a link to the analytics definitions documentation.

This proposal came from @ogolowinski in #300472 (comment 677444470)