docs: Adapt wal_segment_size docs to
fc49e24fa69.
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
fc49e24fa69 itself.