doc: expand note about pg_upgrade's --jobs option

Magnus noted to me off-list that the "et cetera" in the following sentence
in pg_upgrade's docs is doing quite a bit of heavy lifting:

The --jobs 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.

I added the "et cetera" in commit 40e2e5e92b to cover the many follow-up
commits that parallelized various pg_upgrade tasks. I was initially
worried that trying to list all the parallelized stuff would be too
verbose, but looking again, I think all the changes can be grouped into
"gathering cluster information" and "performing cluster checks." The
attached patch replaces the "et cetera" with those two general categories.

--
nathan