pgsql: Doc: fix thinko in description of how to escape a backslash in b

Doc: fix thinko in description of how to escape a backslash in bytea.

Also clean up some discussion that had been left in a very confused
state thanks to half-hearted adjustments for the change to
standard_conforming_strings being the default.

Discussion: /messages/by-id/154954987367.1297.4358910045409218@wrigleys.postgresql.org

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/2bcbf687e4265cad739d7fd6cb5f405bae560ed5

Modified Files
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doc/src/sgml/datatype.sgml | 58 +++++++++++++++++++++-------------------------
1 file changed, 26 insertions(+), 32 deletions(-)