doc: mention datadir locations are actually config locations

Technically, pg_upgrade's --old-datadir and --new-datadir are
configuration directories, not necessarily data directories.  This is
reflected in the 'postgres' manual page, so do the same for pg_upgrade.

Reported-by: Yves Goergen

Bug: 14898

Discussion: https://postgr.es/m/20171110220912.31513.13322@wrigleys.postgresql.org

Backpatch-through: 10