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_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/aee67299306e717986f054d149f6cb2f1c447656

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