Avoid too prominent use of "backup" on pg_dump man page

In Karen Jex's presentation today at pgconf.dev [0] one of the examples
given was that users inadvertently rely on pg_dump as their primary
backup tool, when better solutions exist. The pg_dump man page is
arguably misleading in that it starts with

"pg_dump is a utility for backing up a PostgreSQL database."

I suggest toning this down a little bit. In the attached patch, I have
replaced most uses of "backup" with "export" and added a short note that
pg_dump is not a general-purpose backup tool.

Suggestions for word-smithing welcome.

[0]:
https://www.pgevents.ca/events/pgconfdev2024/schedule/session/126-how-postgres-is-misused-and-abused-in-the-wild/