Add a comment for ColumnsDescription::subcolumns

Signed-off-by: Azat Khuzhin <a.khuzhin@semrush.com>

src/Storages/ColumnsDescription.h

@@ -217,6 +217,13 @@ public:
 
 private:
     ColumnsContainer columns;
+    /// Subcolumns is not nested columns.
+    ///
+    /// Example of subcolumns:
+    /// - .size0 for Array
+    /// - .null  for Nullable
+    ///
+    /// While nested columns is just foo.bar
     SubcolumnsContainter subcolumns;
 
     void modifyColumnOrder(const String & column_name, const String & after_column, bool first);