avcodec/h264_refs: Document last if() in ff_h264_execute_ref_pic_marking()

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 697984b9db)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/h264_refs.c

@@ -807,6 +807,7 @@ int ff_h264_execute_ref_pic_marking(H264Context *h)
         }
     }
 
+    // Detect unmarked random access points
     if (   err >= 0
         && h->long_ref_count==0
         && (   h->short_ref_count<=2