[css-ruby] Auto value of ruby-position

We recently received a request that, by default, position of annotation
should be different for the first and the second rtc of a ruby segment. [1]
I think this request is reasonable, and double-sided ruby is actually the
most common use case for more than one ruby text container. And it is also
something required by the XHTML Ruby spec [2] (though no one implemented
that).

But regarding that the markups can be arbitrary for ruby, it is hard to
express this requirement simply in the default stylesheet. Hence I propose
again to add an "auto" value to ruby-position, which was proposed before
[3], and was not be discussed because of lack of editor. [4]

My proposal is: This value is not computed to anything else. And during the
positioning, this value is equal to "over" for rtc in odd indexes, and
"under" for rtc in even indexes.

Does it sound like something we can add to the spec?

[1] http://momdo.hatenablog.jp/entry/20150219/1424357096
[2] http://www.w3.org/TR/2001/REC-ruby-20010531/#presentation
[3] https://lists.w3.org/Archives/Public/www-style/2009May/0189.html
[4] https://lists.w3.org/Archives/Public/www-style/2009May/0194.html

- Xidorn