[TTML10SE] additional edits - D.1.42 to D.1.50 - #length* features

After some offline discussion with Sean, we felt it important to add some
clarifying notes to D.1.42 to D.1.50 regarding the intended semantics of
the #length* features, since it wasn't clear what there relationship was to
one another or whether there was an implied hierarchy.

Please take a look at these changes at [1] ff.

[1]
https://dvcs.w3.org/hg/ttml/raw-file/default/ttml10/spec/ttaf1-dfxp.html#feature-length