Add maintainer comment in null_restriction.rs

Clarify supported expression/operator families in the syntactic
evaluator. Emphasize that returning None indicates deferral to
authoritative evaluation, rather than "non-restricting." Ensure
unsupported variants also return None for consistency.

Author: kosiew

datafusion/optimizer/src/push_down_filter.rs

@@ -2535,8 +2535,8 @@ mod tests {
     }
 
     #[test]
-    fn scalar_subquery_cross_join_filter_is_treated_as_join_condition_candidate(
-    ) -> Result<()> {
+    fn scalar_subquery_cross_join_filter_is_treated_as_join_condition_candidate()
+    -> Result<()> {
         let predicate = col("s.acctbal").gt(col("__scalar_sq_1.avg_acctbal"));
 
         assert!(can_evaluate_as_join_condition(&predicate)?);

datafusion/optimizer/src/utils/null_restriction.rs

@@ -127,6 +127,15 @@ fn syntactic_null_substitution_value(
     expr: &Expr,
     join_cols: &HashSet<&Column>,
 ) -> Option<NullSubstitutionValue> {
+    // This evaluator intentionally supports a strict subset of expressions:
+    // aliases/columns/literals, boolean combinators (NOT/AND/OR), null checks
+    // (IS [NOT] NULL), BETWEEN, strict-null-preserving unary operators
+    // (CAST/TRY_CAST/NEGATIVE), LIKE/SIMILAR TO, and binary operators handled in
+    // `syntactic_binary_value`.
+    //
+    // Returning `None` means "defer to the authoritative evaluator" rather than
+    // "not null-restricting". Any unsupported expression variant must return
+    // `None` so callers can safely fall back to full expression evaluation.
     match expr {
         Expr::Alias(alias) => {
             syntactic_null_substitution_value(alias.expr.as_ref(), join_cols)