Lua: Improved TreeItem:add documentation.

The protofield is optional and can not be replaced with 'nil'
as with the other fields for this function.

Change-Id: I2b1dd7f290264394b400cea2110b65b657c71456
Reviewed-on: https://code.wireshark.org/review/4549
Reviewed-by: Hadriel Kaplan <hadrielk@yahoo.com>
Tested-by: Hadriel Kaplan <hadrielk@yahoo.com>
Reviewed-by: Stig Bjørlykke <stig@bjorlykke.org>
Tested-by: Stig Bjørlykke <stig@bjorlykke.org>