Doc: clarify explanation of pg_dump usage.

This section confusingly used both "infile" and "outfile" to refer
to the same file, i.e. the textual output of pg_dump.  Use "dumpfile"
for both cases, per suggestion from Jonathan Katz.

Discussion: https://postgr.es/m/152311295239.31235.6487236091906987117@wrigleys.postgresql.org