Document KLL serialization format compatibility guarantee (facebookin…

…cubator#9518)

Summary:
Pull Request resolved: facebookincubator#9518

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Differential Revision: D56264716

fbshipit-source-id: da847fa21c78e858a62e25dfd86e88422ecb31f5

velox/functions/lib/KllSketch.h

@@ -105,11 +105,14 @@ struct KllSketch {
   /// Calculate the size needed for serialization.
   size_t serializedByteSize() const;
 
-  /// Serialize the sketch into bytes.
+  /// Serialize the sketch into bytes.  The serialzation is versioned, and newer
+  /// version of code should be able to read all previous versions.
+  ///
   /// @param out Pre-allocated memory at least serializedByteSize() in size
   void serialize(char* out) const;
 
-  /// Deserialize a sketch from bytes.
+  /// Deserialize a sketch from bytes.  Newer version of code should be able to
+  /// read all previous versions.
   static KllSketch<T, Allocator, Compare> deserialize(
       const char* data,
       const Allocator& = Allocator(),