doc: Add details about object "wal" in pg_stat_io
This commit adds a short description of what kind of activity is tracked
in pg_stat_io for the object "wal", with a link pointing to the section
"WAL configuration" that has a lot of details on the matter.
This should perhaps have been added in
a051e71e28a1, but things are what
they are.
Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Discussion: https://postgr.es/m/Z7RkQ0EfYaqqjgz/@ip-10-97-1-34.eu-west-3.compute.internal