Finish off the incomplete options documentation

The documentation is not completely finished, but now we have no
placeholders waiting to be completed.
Added more documentation

This adds documentation for a `dropdownParent` option which has
not yet been created, but will come in the next commit.
Added `dropdownParent`

`dropdownParent` controls the location of the dropdown when it is
automatically added to the DOM. By default, this is the body
element but it can be modified based on where the user needs it.