docs: add clarification comments for utf8_literal usage in arrow_cast tests

Author: kosiew

python/tests/test_functions.py

@@ -911,6 +911,9 @@ def test_temporal_functions(df):
 
 def test_arrow_cast(df):
     df = df.select(
+        # we use `utf8_literal` to returns utf8 instead of `literal` which returns utf8view
+        # because datafusion.arrow_cast # expects a utf8 instead of utf8view
+        # https://github.com/apache/datafusion/blob/86740bfd3d9831d6b7c1d0e1bf4a21d91598a0ac/datafusion/functions/src/core/arrow_cast.rs#L179
         f.arrow_cast(column("b"), utf8_literal("Float64")).alias("b_as_float"),
         f.arrow_cast(column("b"), utf8_literal("Int32")).alias("b_as_int"),
     )