postgres usage in caveats; Closes #21920

As a PostgreSQL n00b, this is the information I wanted.

Ideally we’d set a default configuration location for postgres to look, however apparently, this is not possible.

Side note: it's about time we split caveats out. One of these is advise for failed builds, and I just added usage. Usage is hardly caveats, but generally usage is cool and we should encourage it as a separate section, and encourage people to add links perhaps.