Add TODO comments for fixing unsupported UPDATE ... FROM syntax

Author: kosiew

datafusion/core/tests/custom_sources_cases/dml_planning.rs

@@ -751,6 +751,7 @@ async fn test_update_from_drops_non_target_predicates() -> Result<()> {
         .await;
 
     // Verify UPDATE ... FROM is rejected with appropriate error
+    // TODO fix https://github.com/apache/datafusion/issues/19950
     assert!(result.is_err());
     let err = result.unwrap_err();
     assert!(

datafusion/sqllogictest/test_files/update.slt

@@ -68,6 +68,7 @@ physical_plan_error This feature is not implemented: Physical plan does not supp
 
 # set from other table
 # UPDATE ... FROM is currently unsupported
+# TODO fix https://github.com/apache/datafusion/issues/19950
 query error DataFusion error: This feature is not implemented: UPDATE ... FROM is not supported
 explain update t1 set b = t2.b, c = t2.a, d = 1 from t2 where t1.a = t2.a and t1.b > 'foo' and t2.c > 1.0;