Added documentation about ramBytesUsed

Author: Mikep86

server/src/main/java/org/elasticsearch/common/bytes/BytesReference.java

@@ -159,7 +159,10 @@ static BytesReference fromByteArray(ByteArray byteArray, int length) {
     BytesReference slice(int from, int length);
 
     /**
-     * The amount of memory used by this BytesReference
+     * The amount of memory used by this BytesReference.
+     * <p>
+     * Note that this is not always the same as length and can vary by implementation.
+     * </p>
      */
     long ramBytesUsed();