[RTA DPA BW Tests] Add comment re: comparison pages showing more than just the MR differences

While writing lhcb/Moore!3837 (merged), decided to add comment directly to the comparison pages

While here, trivially closes lhcb-datapkg/PRConfig#32 (closed) via adding a comment !299 (4340e7d0)

ToDo

• Wait for lhcb/Moore!3837 (merged), maybe more changes to come
• Review