pgsql: Add mention of compatibility problem with turning off backslash

Log Message:
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Add mention of compatibility problem with turning off backslash escapes:

< SQL-spec compliant, so allow such handling to be disabled.

SQL-spec compliant, so allow such handling to be disabled. However,
disabling backslashes could break many third-party applications and tools.

Modified Files:
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pgsql/doc:
TODO (r1.1464 -> r1.1465)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/TODO.diff?r1=1.1464&amp;r2=1.1465)