Update documentation and examples for PGDATA changes

[tianon]

This is a lot more explicit about upgrades instead of relying on implicit image defaults (which change in 18+, so were hard to write generic documentation around anyways).

See also:

• Change PGDATA in 18+ to /var/lib/postgresql/MAJOR/docker docker-library/postgres#1259
• Note on PGDATA changes for PostgreSQL 18+ docker-library/docs#2582
• Update PostgreSQL documentation now that 18 is GA docker-library/docs#2624
• Remove intentionally-breaking "data" symlink and add better detection docker-library/postgres#1372