Merge pull request #741 from w3c/issue-0712-position-example

Clarify explanation of percentage edge offset in tts:position example…

spec/ttml2.xml

@@ -11443,7 +11443,8 @@ the computed position and the supported position is minimized. If there are mult
 the computed value, then the value least distant from [0,0], i.e., closest to the coordinate space position, is used.</p>
 </note>
 <p>The <att>tts:position</att> style is illustrated by the following example, which positions a region so that it is
-centered in the horizontal dimension and has a bottom edge 10% above the bottom of the positioning rectangle in the vertical dimension.</p>
+centered in the horizontal dimension and has a bottom edge offset at 10% above the bottom of the positioning rectangle in the vertical dimension,
+where this offset is equal to 10% of the difference between the height of the positioning rectangle and the height of the region.</p>
 <table id="style-attribute-position-example-1" role="example">
 <caption>Example Fragment &ndash; Position</caption>
 <tbody>