And some lines from selected manuals - you can usually access your car manual if you don't have a physical copy by going to the manufacturer's UK website (be careful not US/Australian) and searching for the model and year.
Car seat manuals lifewise - on the manufacturer's website under support and downloads, or search the model name - retired models should still have downloads available.
Peugeot 3008: (In section: "Seatbelts: Installing the belt") The lower part of the strap must be positioned as low as possible over the pelvis. The upper part must be positioned in the hollow of the shoulder.
Britax: • The more snugly the belt fits over your child's body, the safer your child will be. Therefore, avoid having your child wear thick clothes under the belt.
Cybex: NOTE! In order to gain optimum protection from the
Cybex Aton 5, the shoulder belts c should lie as close as
possible to the body.
Joie: ! DO NOT have your child in large/oversized clothes because this may prevent your child from being properly and securely fastened by the shoulder harness straps and the crotch strap between the legs.
Maxi Cosi (no info)
Cuggl (Argos): In winter, make sure that the child is not wearing over-bulky clothes.
Cosatto: •It is important that the lap harness is worn low down, so that the pelvis of the child is firmly engaged.
Cosatto: In group 2 & 3 modes, the lap vehicle seat belt must be worn in such a way as to properly
secure the pelvis of the child.
Silver Cross: (No info)
It's not always that clear unfortunately. Most of these warnings are also separate from the section about how to put the child in the seat, they are instead situated near the start of the book in a list of generic "warnings". I always wonder how often they get read - it seems like car seat manuals are written more for engineers than parents.