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 
Reviewed-by: Daniel Gustafsson 
Reviewed-by: Magnus Hagander 
Discussion: https://postgr.es/m/Z8dBn_5iGLNuYiPo%40nathan