math.primes.erato: add some comments at the top.
math.primes.erato: add some comments at the top.

nicolas-p/factor,factor/factor,mrjbq7/factor,tgunr/factor,dch/factor,AlexIljin/factor,bpollack/factor,bjourne/factor,mrjbq7/factor,tgunr/factor,bjourne/factor,bjourne/factor,dch/factor,dch/factor,factor/factor,dch/factor,bpollack/factor,AlexIljin/factor,mrjbq7/factor,mrjbq7/factor,AlexIljin/factor,dch/factor,slavapestov/factor,tgunr/factor,tgunr/factor,bpollack/factor,nicolas-p/factor,nicolas-p/factor,bjourne/factor,slavapestov/factor,factor/factor,bjourne/factor,factor/factor,factor/factor,slavapestov/factor,bpollack/factor,bjourne/factor,mrjbq7/factor,factor/factor,dch/factor,nicolas-p/factor,tgunr/factor,slavapestov/factor,nicolas-p/factor,bpollack/factor,slavapestov/factor,bpollack/factor,AlexIljin/factor,mrjbq7/factor,nicolas-p/factor,tgunr/factor,AlexIljin/factor,bjourne/factor,AlexIljin/factor,slavapestov/factor,bpollack/factor,slavapestov/factor,nicolas-p/factor,AlexIljin/factor