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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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/c1769970499833e451d0125ed1bc43d116ad3160

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