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_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/7d6d1555926ae6c5b24016a3399b5cdf73fe712f

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