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>

doc/api/util.md

@@ -551,6 +551,8 @@ util.isUndefined(null)
 
 ## util.log(string)
 
+    Stability: 0 - Deprecated: Use a third party module instead.
+
 Output with timestamp on `stdout`.
 
     require('util').log('Timestamped message.');