Update and fix some issues in EXT-REFERENCE documentation (#5180)

There was some issues and typos in the EXT-REFERENCE documentation.
This patch fixes these issues and typos.

JerryScript-DCO-1.0-Signed-off-by: Gergo Csizi gergocs@inf.u-szeged.hu

docs/09.EXT-REFERENCE-ARG.md

@@ -361,6 +361,28 @@ jerryx_arg_transform_array (const jerry_value_t array_val,
 
 - [jerryx_arg_array](#jerryx_arg_array)
 
+## jerryx_arg_transform_optional
+
+**Summary**
+
+The common function to deal with optional arguments. The core transform function is provided by argument `func`.
+
+**Prototype**
+
+```c
+jerry_value_t jerryx_arg_transform_optional (jerryx_arg_js_iterator_t *js_arg_iter_p,
+                                             const jerryx_arg_t *c_arg_p,
+                                             jerryx_arg_transform_func_t func);
+```
+
+ - `js_arg_iter_p` - the JS arg iterator.
+ - `c_arg_p` - the native arg.
+ - `func` - the core transform function.
+ - return value - a `jerry_value_t` representing `undefined` if all validators passed or an `Error` if a validator failed.
+
+**See also**
+
+- [jerryx_arg_transform_func_t](#jerryx_arg_transform_func_t)
 
 # Helpers for commonly used validations
 
@@ -577,11 +599,11 @@ User should prepare the `jerryx_arg_object_props_t` instance, and pass it to thi
 
 ```c
 static inline jerryx_arg_t
-jerryx_arg_object_properties (const jerryx_arg_object_props_t *object_props_p,
+jerryx_arg_object_properties (const jerryx_arg_object_props_t *obj_prop_p,
                               jerryx_arg_optional_t opt_flag);
 ```
  - return value - the created `jerryx_arg_t` instance.
- - `object_props_p` - provides information for properties transform.
+ - `obj_prop_p` - provides information for properties transform.
  - `opt_flag` - whether the argument is optional.
 
 **Example**
@@ -754,6 +776,23 @@ my_external_handler (const jerry_value_t function_obj,
 
 # Functions to create custom validations
 
+## jerryx_arg_ignore
+
+**Summary**
+
+Create a jerryx_arg_t instance for ignored argument.
+
+**Prototype**
+
+```c
+static inline jerryx_arg_t jerryx_arg_ignore (void);
+```
+ - return value - the created `jerryx_arg_t` instance.
+
+**See also**
+
+- [jerryx_arg_t](#jerryx_arg_t)
+
 ## jerryx_arg_custom
 
 **Summary**