add comments back in

Author: Hernan Gelaf-Romer

hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/RSRpcServices.java

@@ -3984,6 +3984,8 @@ public GetCachedFilesListResponse getCachedFilesList(RpcController controller,
   private QuotaAndRegionScannerHolder checkQuotaAndGetRegionScannerHolder(ScanRequest request,
     ScanResponse.Builder builder) throws IOException {
     if (request.hasScannerId()) {
+      // The downstream projects such as AsyncHBase in OpenTSDB need this value. See HBASE-18000
+      // for more details.
       long scannerId = request.getScannerId();
       RegionScannerHolder rsh = getRegionScanner(request);
       String scannerName = toScannerName(scannerId);