Before this change the docs weren't adapted to the fact that
wal_segment_size is now measured in bytes, rather than multiples of
wal_block_size.

Author: David Steele
Discussion: https://postgr.es/m/68ea97d6-2ed9-f339-e57d-ab3a33caf3b1@pgmasters.net
Backpatch: 11-, like fc49e24f itself.