Improve some comments in pg_upgrade.c

Improve some comments in pg_upgrade.c

When restoring database schemas on a new cluster, database "template1"
is processed first, followed by all other databases in parallel,
including "postgres".  Both "postgres" and "template1" have some extra
handling to propagate each one's properties, but comments were confusing
regarding which one is processed where.

Author: Julien Rouhaud
Reviewed-by: Daniel Gustafsson
Discussion: https://postgr.es/m/CAOBaU_a2iviTG7FE10yO_gcW+zQCHNFhRA_NDiktf3UR65BHdw@mail.gmail.com