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_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/7f39f0344129706979174ba530f8cc21b7d9f4a3

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