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

doc/src/sgml/ref/pgupgrade.sgml

@@ -38,9 +38,9 @@
   <application>pg_upgrade</application> (formerly called <application>pg_migrator</application>) allows data
   stored in <productname>PostgreSQL</productname> data files to be upgraded to a later <productname>PostgreSQL</productname>
   major version without the data dump/reload typically required for
-  major version upgrades, e.g. from 9.6.3 to the current major release
-  of <productname>PostgreSQL</productname>.  It is not required for minor version upgrades, e.g. from
-  9.6.2 to 9.6.3.
+  major version upgrades, e.g. from 9.5.8 to 9.6.4 or from 10.7 to 11.2.
+  It is not required for minor version upgrades, e.g. from 9.6.2 to 9.6.3
+  or from 10.1 to 10.2.
  </para>
 
  <para>