Add Extended range selectors proposal

proposals/2025-04-04_extended-range-selectors-semantics.md

@@ -212,7 +212,7 @@ increase(http_requests_total[5m] anchored)
 
 **Behavior**:
 
-- This mode includes samples within the range `(start, end]` and additionally looks backward to find one sample before the start time.
+- This mode includes samples within the range `(start, end]` and additionally looks backward to find one sample at or before the start time.
 - Staleness markers are ignored.
 - At the start of the range (or evaluation time, for instant selectors),
   Prometheus will search **backward** using a lookback window to find the sample
@@ -270,6 +270,7 @@ Smoothed vectors behave as if continuous lines were drawn between samples, utili
 - If there is no datapoint after the range, the last datapoint inside of the range is duplicated at the end of the range.
 - If there are no datapoints inside the range but samples exist before and after, it interpolates between those samples to estimate values at the range boundaries.
 - The interpolation window is controlled by the **lookback delta**,
+- If the range selector duration is shorter than the evaluation step interval, the calculation will use the step interval as the effective range duration. A warning will then be returned to the user.
 
 
 **Use cases**: