DOCS/man/options: clarify --directory-filter-types docs

Using "mpv --directory-filter-types= dir" as an "empty" value doesn't
work, because that will create a list where the first value is an empty
string, so get_directory_filter() returns AUTO_NONE instead of AUTO_ANY.

You need to use --directory-filter-types-clr instead, to create a proper
empty list.

I suppose this is kind of obvious in hindsight, but it took me quite
some time and digging through the mpv source to figure this out –
initially I assumed it must be a bug of some sort.

So clarify what "empty list" means.