Обсуждение: 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
------
REL9_6_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/385627c2c174e865aa3da1ea220ce8897c9455ba

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