Update documentation for selectedOptions parameter with template support

[sanchesfree]

Added detailed information about the selectedOptions parameter in the documentation, highlighting its ability to support templating (e.g., #cnt# as a placeholder for the count of selected items). This update ensures users are aware of the flexible text customization options available in the multiselect component.

Example usage:

$('.multiselect-element').multiselect({  
    texts: {  
        selectedOptions: 'Selected: #cnt#',  
    }  
});