Add notes to jerry_create_undefined

JerryScript-DCO-1.0-Signed-off-by: Mate Dabis mdabis@inf.u-szeged.hu

Author: matedabis

docs/02.API-REFERENCE.md

@@ -8176,6 +8176,11 @@ main (void)
 
 Creates a `jerry_value_t` representing an undefined value.
 
+*Notes*:
+- Calling [jerry_release_value](#jerry_release_value) is optional.
+- Calling [jerry_acquire_value](#jerry_acquire_value) is optional.
+
+
 **Prototype**
 
 ```c