Document units for max_slot_wal_keep_size
The doc blurb failed to mention units, as well as lacking the point
about changeability.
Backpatch to 13.
Reviewed-by: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Reported by:
b1000101@pm.me
Discussion: https://postgr.es/m/
163760291192.26193.
10801700492025355788@wrigleys.postgresql.org