ACTION-84: Create clarifying text for 2nd edition for cell units

Clarify feature described in Issue:72. Separate topic of smooth scrolling and dynamic flow. Sean will provide clarifying text on the latter. See minutes from Aug 16: http://www.w3.org/2012/08/16-tt-minutes.html

Text from Aug 16 minutes on this action and ISSUE-72: mike: Dynamic flow

sean: Not supported in TTML 1.0. We don't have scroll property any longer. Fixed mechanism exists - overflow.

Missing text will overflow or be clipped by the box.

Proposal suggests we re-add scroll property.

mike: Need to address what happens when you have a <br>.

sean: Some text will be clipped out. Allow animation using inline scroll (XSL-FO). To preclude lines missing from flow region.

mike: In 2nd edition, we need to clarify feature and consider for TTML 1.1.

sean: Clarifying paragraphs.

Correction, this is for Issue-163. Another action will be created for dynamic flow.

ISSUE-163?

<trackbot> ISSUE-163 -- Clarify use of cell units in certain properties -- open

<trackbot> http://www.w3.org/AudioVideo/TT/tracker/issues/163

sean: Use cell values where they do not make sense.
... Issue for 2nd edition.

Clarify semantics.

<scribe> ACTION: Sean create clarifying text for 2nd edition. [recorded in http://www.w3.org/2012/08/16-tt-minutes.html#action04]

<trackbot> Created ACTION-84 - Create clarifying text for 2nd edition. [on Sean Hayes - due 2012-08-23].

added clarifying note at end of 6.2.1