Add some commentary re: fix to #11203
The fix for #11203 prohibits duplicate SigTvs in non-CUSK kind signatures by checking for duplicates after type inference is done. This works well. GHC also checks for duplicate SigTvs after working with partial type signatures (another place where SigTvs arise). However, neither fix eliminates this whole class of problems (because doing so would be heavier than we would like). So, this comment adds a warning to users of newSigTyVar to be aware of problems with duplicates.