doc: Recommend pg_stat_io rather than pg_stat_wal in WAL configuration

doc: Recommend pg_stat_io rather than pg_stat_wal in WAL configuration

Since a051e71e28a1, pg_stat_io is able to track statistics for the WAL
activity, providing an equivalent of pg_stat_wal with more granularity
for the fsyncs/writes counts and timings, as the data is split across
backend types.

This commit now recommends pg_stat_io rather than pg_stat_wal in the
section "WAL configuration", some of the latter's attributes being
candidate for removal in a follow-up commit.

Extracted from a larger patch by the same author.

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Discussion: https://postgr.es/m/Z7RkQ0EfYaqqjgz/@ip-10-97-1-34.eu-west-3.compute.internal