doc: doc-only deprecation for util.log()

There are more powerful loggers in user land like `debug`, soft
deprecate it.

PR-URL: https://github.com/nodejs/node/pull/6161


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Evan Lucas <evanlucas@me.com>