fix: add explanatory comment for schema in Connect xml handler

Author: Yicong-Huang

python/pyspark/sql/connect/readwriter.py

@@ -443,6 +443,8 @@ def xml(
         from pyspark.sql.connect.dataframe import DataFrame
 
         if isinstance(path, DataFrame):
+            # Schema must be set explicitly here because the DataFrame path
+            # bypasses load(), which normally calls self.schema(schema).
             if schema is not None:
                 self.schema(schema)
             return self._df(