docs: Link missing proto fields to github issue in TODO comment

Author: geoffreyclaude

datafusion/proto/src/logical_plan/from_proto.rs

@@ -301,7 +301,8 @@ pub fn parse_expr(
                     exec_datafusion_err!("missing window frame during deserialization")
                 })?;
 
-            // TODO: support proto for null treatment
+            // TODO: support null treatment, distinct, and filter in proto.
+            // See https://github.com/apache/datafusion/issues/17417
             match window_function {
                 window_expr_node::WindowFunction::Udaf(udaf_name) => {
                     let udaf_function = match &expr.fun_definition {

datafusion/proto/src/logical_plan/to_proto.rs

@@ -314,7 +314,8 @@ pub fn serialize_expr(
                         ref partition_by,
                         ref order_by,
                         ref window_frame,
-                        // TODO: support null treatment in proto
+                        // TODO: support null treatment, distinct, and filter in proto.
+                        // See https://github.com/apache/datafusion/issues/17417
                         null_treatment: _,
                         distinct: _,
                         filter: _,