pgsql: doc: \123 and \x12 escapes in COPY are in database encoding.

doc: \123 and \x12 escapes in COPY are in database encoding.

The backslash sequences, including \123 and \x12 escapes, are interpreted
after encoding conversion. The docs failed to mention that.

Backpatch to all supported versions.

Reported-by: Andreas Grob
Discussion: https://www.postgresql.org/message-id/17142-9181542ca1df75ab%40postgresql.org

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/b889d6c7196d120c9e9a357dea5cd77d68018600

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