[css-lists-3] Clarify effect of list styling on speech output

[fantasai]

We don't talk about what gets read out for list styling, so we need to specify that more clearly. (It kinda can be argued to fall out from the definition of the content property and @counter-style’s speak-as descriptor but we need to be clearer about it.)

Additional discussion on this topic that should probably be triaged:

• thread at http://www.w3.org/mid/E17F75B6E86AE842A57B4534F82D0376CD2444@MTAMAIL.muni.sfgov.org [runs through May]