[DOCS] log_lock_waits does not document requiring superuser rights

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/9.6/static/runtime-config-logging.html
Description:

For most settings
https://www.postgresql.org/docs/9.6/static/runtime-config-logging.html says
"Only superusers can change this setting."

It does not say this for "log_lock_waits", but superuser rights are still
required (tested on 9.6.3, but I think it's always been the case).