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: https://postgr.es/m/CALDaNm3zWr=OmxeNqOqfT=uZTSdam_j-gkX94CL8eTNfgUtf6A@mail.gmail.com
Backpatch-through: 12

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/580d8ac9b1ef2803f39b1e23d9da68fff9109c42

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