pgsql: doc: Fix description of \copy for psql

doc: Fix description of \copy for psql

The WHERE clause introduced by 31f3817 was not described. While on it,
split the grammar of \copy FROM and TO into two distinct parts for
clarity as they support different set of options.

Author: Vignesh C
Discussion: /messages/by-id/CALDaNm3zWr=OmxeNqOqfT=uZTSdam_j-gkX94CL8eTNfgUtf6A@mail.gmail.com
Backpatch-through: 12

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b74d449a02b3c972051b1847f3915128da8775dc

Modified Files
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doc/src/sgml/ref/psql-ref.sgml | 10 ++++++++--
1 file changed, 8 insertions(+), 2 deletions(-)