Prometheus: Invest in tests and docs for min interval or min step explanation (#80165)

* add clearer comment for function def

* update test to reflect change in range for 1w step

* clarify docs

* add more clarity

* add explanation to query options min interval and link to min step

* Update docs/sources/panels-visualizations/query-transform-data/_index.md

Co-authored-by: Isabel <76437239+imatwawana@users.noreply.github.com>

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Co-authored-by: Isabel <76437239+imatwawana@users.noreply.github.com>

pkg/tsdb/prometheus/models/query.go

@@ -284,6 +284,11 @@ func isVariableInterval(interval string) bool {
 	return false
 }
 
+// This function aligns query range to step and handles the time offset.
+// It rounds start and end down to a multiple of step.
+// Prometheus caching is dependent on the range being aligned with the step.
+// Rounding to the step can significantly change the start and end of the range for larger steps, i.e. a week.
+// In rounding the range to a 1w step the range will always start on a Thursday.
 func AlignTimeRange(t time.Time, step time.Duration, offset int64) time.Time {
 	offsetNano := float64(offset * 1e9)
 	stepNano := float64(step.Nanoseconds())