diff --git a/cpp/src/arrow/testing/random.h b/cpp/src/arrow/testing/random.h
index 64d02023129f4..269ce3eabf6b4 100644
--- a/cpp/src/arrow/testing/random.h
+++ b/cpp/src/arrow/testing/random.h
@@ -468,9 +468,10 @@ class ARROW_TESTING_EXPORT RandomArrayGenerator {
 
   /// \brief Generate a random SparseUnionArray
   ///
-  /// The type ids are chosen randomly, according to a uniform distribution,
-  /// amongst the given child fields.
+  /// The type ids are chosen randomly from `type.type_codes()`, according to a uniform
+  /// distribution, amongst the given child fields.
   ///
+  /// \param[in] type The union data type
   /// \param[in] fields Vector of Arrays containing the data for each union field
   /// \param[in] size The size of the generated sparse union array
   /// \param[in] alignment alignment for memory allocations (in bytes)
@@ -482,10 +483,11 @@ class ARROW_TESTING_EXPORT RandomArrayGenerator {
 
   /// \brief Generate a random DenseUnionArray
   ///
-  /// The type ids are chosen randomly, according to a uniform distribution,
-  /// amongst the given child fields.  The offsets are incremented along
+  /// The type ids are chosen randomly from `type.type_codes()`, according to a uniform
+  /// distribution, amongst the given child fields.  The offsets are incremented along
   /// each child field.
   ///
+  /// \param[in] type The union data type
   /// \param[in] fields Vector of Arrays containing the data for each union field
   /// \param[in] size The size of the generated sparse union array
   /// \param[in] alignment alignment for memory allocations (in bytes)