[UI] Improve UPnP explanation on preference page "Network"

Change-Id: If1bc88cbcaacb1dc21a070a1c5aecdd3088df9c1
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1454
Tested-by: Jenkins CI
Reviewed-by: Bernd Bieber <brotlee@zedat.fu-berlin.de>
Reviewed-by: Andrej Szaffranietz <a.szaffranietz@gmail.com>
Reviewed-by: Arndt Lasarzik <alasarzik@gmail.com>
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>