[i18n-activity] tts:displayAlign should mention right-to-left effects

aphillips has just created a new issue for 
https://github.com/w3c/i18n-activity:

== tts:displayAlign should mention right-to-left effects ==
https://w3c.github.io/ttml2/spec/ttml2.html#style-attribute-displayAlign

The style `tts:displayAlign` is written in such a way as to make 
`tts:direction=rtl` change what the keywords `before` and `after` 
mean. It says (in part):

> The tts:displayAlign attribute is used to specify a style property 
that defines the alignment of block areas in the block progression 
direction.

However, none of the examples include setting `tts:direction` or 
mention the effect of RTL presentation. Suggest that there be text 
that mentions this behavior.

Please view or discuss this issue at 
https://github.com/w3c/i18n-activity/issues/330 using your GitHub 
account