Add more explanation to the comments

cpp/src/structs/utilities.cpp

@@ -95,10 +95,16 @@ struct flattened_table {
   {
     // Even if it is not required to extract the bitmask to a separate column,
     // we should always do that if the structs column has any null element.
+    //
     // In addition, we should check for null by calling to `has_nulls()`, not `nullable()`.
     // This is because when comparing structs columns, if one column has bitmask while the other
     // does not (and both columns do not have any null element) then flattening them using
     // `nullable()` will result in tables with different number of columns.
+    //
+    // Notice that, for comparing structs columns when one column has null while the other
+    // doesn't, `nullability` must be passed in with value `column_nullability::FORCE` to make
+    // sure the flattening results are tables having the same number of columns.
+
     if (nullability == column_nullability::FORCE || col.has_nulls()) {
       validity_as_column.push_back(cudf::is_valid(col));
       if (col.has_nulls()) {