Constants are variables that cannot change. They are constant.
In this case they are variables that have been set up in the board definition files (in pins_arduino.h in case you're interested) and are named just as they have said - A6 through A11. This complements the existing A0 through A5 that are normally there anyway.
So you just use those names and it chooses the right pin for you:
int val = analogRead(A8);