[RTADPA BW Tests] Add disclaimer to PbPb bandwidth test about inaccurate rates and bandwidth

Add a warning to the PbPb bandwidth test that absolute rates and bandwidths are not accurate.

Resolves lhcb-datapkg/PRConfig#46 (closed), final comment in lhcb-datapkg/PRConfig!602 (comment 10306301).

Pages https://jconnaug-bwtests.web.cern.ch/Moore5468_add_PbPb_warning/