Update TextArea::wrapMode docs

The documentation adapted from TextEdit mentioned a wrong default
value and contained confusing and unnecessary low-level details how
TextEdit calculates its content/implicitWidth. TextArea hides these
details and the documentation has been simplified accordingly.

Task-number: QTBUG-38022
Change-Id: Ifa29956b59b7f169fbdc94f455dbdad29469db3e
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>