add comments for ivfflat getIteratorNext()

Signed-off-by: min.tian <[email protected]>

thirdparty/faiss/faiss/IndexIVFFlat.h

@@ -53,7 +53,7 @@ struct IndexIVFFlat : IndexIVF {
     void restore_codes(const uint8_t* raw_data, const size_t raw_size);
 
     // Be careful with overriding this function, because
-    //   renormalized x may be used inside. 
+    //   renormalized x may be used inside.
     // Overridden by IndexIVFFlatDedup.
     void train(idx_t n, const float* x) override;
 
@@ -88,6 +88,12 @@ struct IndexIVFFlat : IndexIVF {
             const float* query_data,
             const IVFSearchParameters* ivfsearchParams) const;
 
+    // Unlike regular knn-search, the iterator does not know the size `k` of the
+    // returned result.
+    //   The workspace will maintain a heap of at least (nprobe/nlist) nodes for
+    //   iterator `Next()` operation.
+    //   When there are not enough nodes in the heap, iterator will scan the
+    //   next coarse list.
     std::optional<std::pair<float, idx_t>> getIteratorNext(
             IVFFlatIteratorWorkspace* workspace) const;
 };