doc: move mention of log_min_error_statement in a better spot
Previously it was mentioned in the lock_timeout docs in a confusing
location.
Reported-by: ivaylo.zlatanov@gmail.com
Discussion: https://postgr.es/m/
157019615723.25307.
15449102262106437404@wrigleys.postgresql.org
Backpatch-through: 9.4