[selectors-4] Add its:dir as an example of markup mapping to :dir()

[fantasai]

Filing based on @js-choi's comment in #2091 (comment)

This again does not apply to arbitrary XML, which needs directionality to style whatever bidi markup it uses, just like HTML’s user-agent style sheet does. I don’t know yet whether a directionality: auto would be appropriate for arbitrary XML…Maybe the real issue is that :dir is not well defined for arbitrary XML, but that also has no perfect solutions: coupling :dir to XML with ITS attributes requires blessing a non-core namespace…and creating an xml:dir attribute would have its own compatibility problems…

We should add its:dir as an example of markup that, if understood by the UA, should be mapped to :dir().