Improve documentation for pg_upgrade, standbys and rsync

I revently tried to upgrade a standby following the documentation,
but I found it hard to understand, and it took me several tries to
get it right. This is of course owing to my lack of expertise with
rsync, but I think the documentation and examples could be clearer.

I think it would be a good idea to recommend the --relative option
of rsync.

Here is a patch that does that, as well as update the versions in
the code samples to something more recent. Also, I think it makes
sense to place the data directory in the sample in /var/lib/postgresql,
which is similar to what many people will have in real life.

Yours,
Laurenz Albe