Help: avoid opening external browser for 'no documentation available'

Task-number: QTCREATORBUG-10513

Change-Id: I07af69fb5223faf5e030f30280ae3125731b79bc
Reviewed-by: Robert Loehning <robert.loehning@digia.com>
Reviewed-by: Karsten Heimrich <karsten.heimrich@digia.com>