add comment describing difference between CovidParams and CovidConstrainedParams
add comment describing difference between
CovidParams and CovidConstrainedParams

OurWorldInData/owid-grapher,owid/owid-grapher,owid/owid-grapher,owid/owid-grapher,OurWorldInData/owid-grapher,owid/owid-grapher,owid/owid-grapher,OurWorldInData/owid-grapher,OurWorldInData/owid-grapher,OurWorldInData/owid-grapher