docs: clarify requestStart behavior for canceled requests in Performance API (#41704)

Update documentation to clarify that canceled requests are still recorded by the Performance API, but may have requestStart value of 0, which can lead to calculation errors in performance metrics.

Author: fengyinxqy

files/en-us/web/api/performanceresourcetiming/requeststart/index.md

@@ -19,6 +19,7 @@ The `requestStart` property can have the following values:
 - A {{domxref("DOMHighResTimeStamp")}} representing the time immediately before the browser starts requesting the resource from the server.
 - `0` if the resource was instantaneously retrieved from a cache.
 - `0` if the resource is a cross-origin request and no {{HTTPHeader("Timing-Allow-Origin")}} HTTP response header is used.
+- `0` if the resource is a canceled request.
 
 When the {{domxref("PerformanceResourceTiming.firstInterimResponseStart", "firstInterimResponseStart")}} is non-zero, that indicates it should be the same value as `requestStart` for [supporting browsers](#browser_compatibility).