Fix documentation for VideoQualityAnalyzerInterface::GetStreamLabel

Bug: b/205824594
Change-Id: I76eff28984446ed94d701129d63f2a1643f9d983
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/238161
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35365}

api/test/video_quality_analyzer_interface.h

@@ -147,6 +147,9 @@ class VideoQualityAnalyzerInterface
   // statistics.
   virtual void Stop() {}
 
+  // Returns the last stream where this frame was captured. It means that if
+  // frame ids space wraps around, then stream label for frame id may change.
+  // It will crash, if the specified `frame_id` wasn't captured.
   virtual std::string GetStreamLabel(uint16_t frame_id) = 0;
 };