pgsql: Fix documentation for dblink_error_message() return value

Fix documentation for dblink_error_message() return value

The dblink documentation claims that an empty string is returned if there
has been no error, however OK is actually returned in that case. Also,
clarify that an async error may not be seen unless dblink_is_busy() or
dblink_get_result() have been called first.

Backpatch to all supported branches.

Reported-by: realyota
Backpatch-through: 9.4
Discussion: https://postgr.es/m/153371978486.1298.2091761143788088262@wrigleys.postgresql.org

Branch
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REL9_5_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/916b50a5ae0a7a64ca3b2497ccb1b97ce72f8aa9

Modified Files
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doc/src/sgml/dblink.sgml | 16 +++++++++++++++-
1 file changed, 15 insertions(+), 1 deletion(-)