doc: Adjust note about pg_upgrade's --jobs option.

Presently, this section lists a couple of parallelized parts of
pg_upgrade and suggests a starting point for setting the --jobs
option.  The list of parallelized tasks is not particularly
actionable, and the phrasing for the --jobs recommendation is
confusing to some readers.

This commit attempts to improve this section by eliminating the
list of parallelized tasks and instead highlighting that --jobs is
most useful for clusters with multiple databases or tablespaces.
Additionally, the recommendation for setting --jobs is simplified
to suggest starting with the number of CPU cores.

Reported-by: Magnus Hagander <magnus@hagander.net>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Magnus Hagander <magnus@hagander.net>
Discussion: https://postgr.es/m/Z8dBn_5iGLNuYiPo%40nathan

Author: nathan-bossart

doc/src/sgml/ref/pgupgrade.sgml

@@ -533,12 +533,11 @@ NET STOP postgresql-&majorversion;
     </para>
 
     <para>
-     The <option>--jobs</option> option allows multiple CPU cores to be used
-     for copying/linking of files, dumping and restoring database schemas
-     in parallel, etc.;  a good place to start is the maximum of the number of
-     CPU cores and tablespaces.  This option can dramatically reduce the
-     time to upgrade a multi-database server running on a multiprocessor
-     machine.
+     Setting <option>--jobs</option> to 2 or higher allows pg_upgrade to
+     process multiple databases and tablespaces in parallel.  A good starting
+     point is the number of CPU cores on the machine.  This option can
+     substantially reduce the upgrade time for multi-database and
+     multi-tablespace servers.
     </para>
 
     <para>