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

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

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/3c472a18296e473270b6ff611b898592263a6ed1

Modified Files
--------------
doc/src/sgml/ref/pgupgrade.sgml | 11 +++++------
1 file changed, 5 insertions(+), 6 deletions(-)