Related fdo#47802: Bring the 8bit and 16bit .doc on par.

The code expects that sTxt holds both the cbTextBefore and cbTextAfter, so
let's make it so in the 16bit format too.

This fixes assertion in doc/fdo47802-3.doc.

Change-Id: I35cc7c1e80a33f013ab2eaae073b111214245e25