doc: Avoid too prominent use of "backup" on pg_dump man page
Some 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."
This tones this down a little bit, by replacing most uses of "backup"
with "export" and adding a short note that pg_dump is not a
general-purpose backup tool.
Discussion: https://www.postgresql.org/message-id/flat/
70b48475-7706-4268-990d-
fd522b038d96%40eisentraut.org