Doc: Update Button::isDefault documentation
Doc: Update Button::isDefault documentation

Change-Id: Icda0e032122bff59d1a1691a2a475a0ecef80dc9
Reviewed-by: Jens Bache-Wiig <9cea9d70b08f8210134ec5959114f72bad51ed1d@digia.com>
Reviewed-by: Nico Vertriest <4c665cb9ab44b1d76bb1eeeb8c58a9dc3bfd72ce@digia.com>

kobolabs/qtquickcontrols,mizukusak/qtquickcontrols,mizukusak/qtquickcontrols,richardmg/qtcomponents,mizukusak/qtquickcontrols,richardmg/qtcomponents,richardmg/qtcomponents,drgvond/qtquickcontrols,kobolabs/qtquickcontrols,drgvond/qtquickcontrols,drgvond/qtquickcontrols,kobolabs/qtquickcontrols