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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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/087c1a2f119b3e27ef989d414750709f7bef3cb8

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