doc: clarify major/minor pg_upgrade versions with examples

The previous docs added in PG 10 were not clear enough for someone who
didn't understand the PG 10 version change, so give more specific
examples.

Reported-by:  <jim@room118solutions.com>

Discussion: https://postgr.es/m/20171218213041.25744.8414@wrigleys.postgresql.org

Backpatch-through: 10