Document accepted values for $parents argument of get_the_category_list()

[coffee2code]

The $parents argument of get_the_category_list() recognizes only 3 acceptable values, none of which are documented.

Patch attached that explicitly lists the 3 acceptable values and the default.

As a follow-on, ideally the description for the parameter or even the function should actually explain the difference between the three options.

[coffee2code]

Patch mentioned in ticket.