bgwriter_lru_maxpages range in postgresql.conf

With v10, the upper limit on bgwriter_lru_maxpages was changed from 1000 to
INT_MAX / 2, but the postgresql.conf.sample was not updated.

#bgwriter_lru_maxpages = 100 # 0-1000 max buffers written/round

I don't see any precedence for including INT_MAX-type limits in the sample
config file, so maybe something like this:

#bgwriter_lru_maxpages = 100 # max buffers written/round, 0 to turn
off

Cheers,

Jeff