Improve pg_dump/pg_dumpall help synopses and terminology

Improve pg_dump/pg_dumpall help synopses and terminology

Increase consistency of --help and man page synopses between pg_dump
and pg_dumpall.  These should now be very similar, as pg_dumpall can
now also produce non-text dump output.  But actually, they had drifted
further apart.

- Use verb "export" consistently, instead of "dump" or "extract".
- Use "SQL script" instead of just "script" or "text file".
- Maintain consistent distinction between SQL script and other
  formats/archives (which is relevant for pg_restore).

Reviewed-by: Robert Treat <rob@xzilla.net>
Discussion: https://www.postgresql.org/message-id/flat/3f71d8a7-095b-4829-9b0b-fce09e9866b3%40eisentraut.org