doc: Move documentation of md5_password_warnings to a better place

doc: Move documentation of md5_password_warnings to a better place

Commit db6a4a985bc categorized md5_password_warnings as an
authentication setting, and the placement in postgresql.conf.sample
matches that, but in the documentation it ended up under logging
settings, which isn't unreasonable but inconsistent.  This moves the
documentation chunk to authentication settings as well.