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