opcua: add hint to OPCUA settings tooltip

It's unclear how the ports should be separated, this additional text in the tooltip clarifies it

Change-Id: I686482522d006024f920a3cc26b83cd21c19f8a5
Reviewed-on: https://code.wireshark.org/review/4147
Reviewed-by: Michael Mann <mmann78@netscape.net>
(cherry picked from commit 17c14d6499c18fa67875c76fb377b4722035a160)
Reviewed-on: https://code.wireshark.org/review/4239