Note this policy experimental

Author: Xiaofei Wang

include/pybind11/detail/common.h

@@ -461,13 +461,16 @@ enum class return_value_policy : uint8_t {
         return_value_policy::reference and the keep_alive call policy */
     reference_internal,
 
-    /** With this policy, C++ string types are converted to Python bytes,
+    /** This policy is experimental and likely to change in the future. With
+        this policy, C++ string types are converted to Python bytes,
         instead of str. This is most useful when a C++ function returns a
         container-like type with nested C++ string types, and py::bytes cannot
-        be applied easily. Note that this return_value_policy is not concerned
-        with lifetime/ownership semantics, like the other policies, but the
-        purpose of return_as_bytes is certain to be orthogonal, because C++
-        strings are always copied to Python bytes or str. */
+        be applied easily. Dictionary like types might not work, for example,
+        `Dict[str, bytes]`, because this policy forces all string return values
+        to be converted to bytes. Note that this return_value_policy is not
+        concerned with lifetime/ownership semantics, like the other policies,
+        but the purpose of return_as_bytes is certain to be orthogonal, because
+        C++ strings are always copied to Python bytes or str. */
     return_as_bytes
 };