Added the _restore function to "unpop" the argument stack. (#2592)

Added a unit test for the jerryx_arg_js_iterator_restore function.
Added the specification to the documentation.

JerryScript-DCO-1.0-Signed-off-by: Timothy Harvey t-harvey@ti.com

docs/09.EXT-REFERENCE-ARG.md

@@ -61,7 +61,7 @@ typedef struct
 
 **Summary**
 
-The structure is used in `jerryx_arg_array`. It provides the array items' corresponding 
+The structure is used in `jerryx_arg_array`. It provides the array items' corresponding
 JS-to-C mappings and count.
 
 **Prototype**
@@ -813,6 +813,32 @@ jerryx_arg_js_iterator_peek (jerryx_arg_js_iterator_t *js_arg_iter_p)
  - return value - the current `jerry_value_t` argument.
  - `js_arg_iter_p` - the JS arg iterator from which to peek.
 
+## jerryx_arg_js_iterator_restore
+
+**Summary**
+
+Restore the last item popped from the stack.  This can be called as
+many times as there are arguments on the stack -- if called when the
+first element in the array is the current top of the stack, this
+function does nothing.
+
+*Note:* This function relies on the underlying implementation of the
+arg stack as an array, as its function is to simply back up the "top
+of stack" pointer to point to the previous element of the array.
+
+*Note:* Like `jerryx_arg_js_iterator_pop ()`, this function will
+change the `js_arg_idx` and `js_arg_p` values in the iterator.
+
+**Prototype**
+
+```c
+jerry_value_t
+jerryx_arg_js_iterator_restore (jerryx_arg_js_iterator_t *js_arg_iter_p)
+```
+ - return value - the the new top of the stack.
+ - `js_arg_iter_p` - the JS arg iterator to restore.
+
+
 ## jerryx_arg_js_iterator_index
 
 **Summary**