Update MergeTreeIOSettings.h

src/Storages/MergeTree/MergeTreeIOSettings.h

@@ -14,6 +14,7 @@ struct MergeTreeReaderSettings
     /// If save_marks_in_cache is false, then, if marks are not in cache,
     ///  we will load them but won't save in the cache, to avoid evicting other data.
     bool save_marks_in_cache = false;
+    /// Convert old-style nested (single arrays with same prefix, `n.a`, `n.b`...) to subcolumns of data type Nested.
     bool convert_nested_to_subcolumns = false;
 };