docs: clarify handling of column lists in COPY TO/FROM
Previously it was unclear how COPY FROM handled cases where not all
columns were specified, or if the order didn't match.
Reported-by: pavlo.golub@gmail.com
Discussion: https://postgr.es/m/
157487729344.7213.
14245726713444755296@wrigleys.postgresql.org
Backpatch-through: 9.4