Add comment for getFreq.

Author: henningandersen

x-pack/plugin/blob-cache/src/main/java/org/elasticsearch/blobcache/shared/SharedBlobCacheService.java

@@ -1663,6 +1663,7 @@ public void close() {
             decayAndNewEpochTask.close();
         }
 
+        // used by tests
         int getFreq(CacheFileRegion<KeyType> cacheFileRegion) {
             return keyMapping.get(cacheFileRegion.regionKey.file().shardId(), cacheFileRegion.regionKey).freq;
         }