[tune] use dated experiment dir per default (#11104)

doc/source/tune/user-guide.rst

@@ -579,6 +579,9 @@ These are the environment variables Ray Tune currently considers:
   ``~/ray_bootstrap_key.pem`` will be used.
 * **TUNE_DISABLE_AUTO_INIT**: Disable automatically calling ``ray.init()`` if
   not attached to a Ray session.
+* **TUNE_DISABLE_DATED_SUBDIR**: Tune automatically adds a date string to experiment
+  directories when the name is not specified explicitly or the trainable isn't passed
+  as a string. Setting this environment variable to ``1`` disables adding these date strings.
 * **TUNE_DISABLE_STRICT_METRIC_CHECKING**: When you report metrics to Tune via
   ``tune.report()`` and passed a ``metric`` parameter to ``tune.run()``, a scheduler,
   or a search algorithm, Tune will error