Add explanation to lifetime event count on issue details

Right now, there are two event counts on issue details, one for the # of events matching the filters and one for # of events in the lifetime of the issue. For the user count we have a label (90 days/max retention window), but the event count has no explanation which causes some confusion. From https://github.com/getsentry/sentry/issues/90547#issuecomment-2843739236