document ReRanker change

UPGRADING.md

@@ -2,11 +2,17 @@
 
 ## Primary API changes
 
-- GraphIndexBuilder M parameter now represents the maximum degree of the graph,
+- `GraphIndexBuilder` `M` parameter now represents the maximum degree of the graph,
   instead of half the maximum degree.  (The former behavior was motivated by making
   it easy to make apples-to-apples comparisons with Lucene HNSW graphs.)  So,
   if you were building a graph of M=16 with JVector2, you should build it with M=32
   with JVector3.
+- `NodeSimilarity.ReRanker` api has changed.  The interface is no longer parameterized,
+  and the `similarityTo` method no longer takes a Map parameter (provided by `search` with
+  the full vectors associated with the nodes returned).  This is because we discovered that
+  (in contrast with the original DiskANN design) it is more performant to read vectors lazily 
+  from disk at reranking time, since this will only have to fetch vectors for the topK nodes 
+  instead of all nodes visited.
 
 ## Other changes to public classes