ve.ce.getDomText: Document node types and use .data instead of .nodeValue

I thought maybe it was using nodeValue as a shortcut for two
different properties between Text and CDATASection nodes, however
they both inherit from CharacterData and store it in .data.

* CDataSection (pre-HTML5)
  http://www.w3.org/TR/2004/REC-DOM-Level-3-Core-20040407/core.html#ID-667469212

* Node.CDATA_SECTION_NODE
  http://dom.spec.whatwg.org/#node

Change-Id: Ib4cd4417fd2b18b123f7bb78d1448d57fa9d78cc