To allow multiple selections in php you need to name your form element: name="pendingdocs[]" (with the brackets).
That will pass the selected values as an array.
How did you want them inserted? Comma-separated? That would be done by updating line 167 to:
$WA_fieldValuesStr = "Y" . $WA_AB_Split . "".((isset($_POST["pendingdocs"]))?implode(", ",$_POST["pendingdocs"]):"") ."";