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

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