Minor fix in API reference (#1536)

JerryScript-DCO-1.0-Signed-off-by: László Langó llango.u-szeged@partner.samsung.com

docs/02.API-REFERENCE.md

@@ -2817,8 +2817,9 @@ jerry_free_property_descriptor_fields (const jerry_property_descriptor_t *prop_d
 
 **Summary**
 
-Call function specified by a function value. Error flag
-must not be set for any arguments of this function.
+Call function specified by a function value. Error flag must
+not be set for any arguments of this function. Value of `this`
+parameter should be set to `undefined` for non-method calls.
 
 *Note*: Returned value must be freed with [jerry_release_value](#jerry_release_value) when it
 is no longer needed.
@@ -2860,6 +2861,8 @@ jerry_call_function (const jerry_value_t func_obj_val,
     jerry_release_value (ret_val);
     jerry_release_value (this_val);
   }
+
+  jerry_release_value (val);
 }
 ```