Update some comments

cpp/src/arrow/compute/kernels/vector_placement_test.cc

@@ -372,7 +372,7 @@ void DoTestPermuteAAA(const std::shared_ptr<Array>& values,
 }
 
 /// The following helper functions are based on the invariant:
-/// PermuteXXA([V, V], [I', I''], 2 * l) == Concat(E, E)
+/// Permute([V, V], [I', I''], 2 * l) == Concat(E, E)
 ///
 /// where
 /// V = values
@@ -1008,6 +1008,9 @@ void DoTestIfElse(const Expression& cond, const Expression& if_true,
  ASSERT_TRUE(result_by_expr.is_array());
  ASSERT_OK_AND_ASSIGN(Datum result_by_permute,
  ExecuteIfElseByPermute(cond, if_true, if_false, schema, input));
+ // Permute will output chunked array because we inputs values and indices as chunked
+ // arrays consisting of each branches. We don't care the shape of the output when
+ // comparing the results - only contents, so we concatenate the chunked array.
  ASSERT_TRUE(result_by_permute.is_chunked_array());
  ASSERT_OK_AND_ASSIGN(auto result_by_permute_concat,
  Concatenate(result_by_permute.chunked_array()->chunks()));