[Runtime Env]Add usage of expansion of certain PATH variables in env_…

…vars (#34856)

Add usage of expansion of certain PATH variables in env vars to documentation

Signed-off-by: LarryLian <554538252@qq.com>

doc/source/ray-core/handling-dependencies.rst

@@ -371,10 +371,15 @@ The ``runtime_env`` is a Python dictionary or a Python class :class:`ray.runtime
   Furthermore, referencing local files `within` a `environment.yml` file is not supported.
 
 - ``env_vars`` (Dict[str, str]): Environment variables to set.  Environment variables already set on the cluster will still be visible to the Ray workers; so there is
-  no need to include ``os.environ`` or similar in the ``env_vars`` field.
+  no need to include ``os.environ`` or similar in the ``env_vars`` field.   
+  By default, these environment variables override the same name environment variables on the cluster. 
+  You can also reference existing environment variables using ${ENV_VAR} to achieve the appending behavior.
+  Only PATH, LD_LIBRARY_PATH, DYLD_LIBRARY_PATH, and LD_PRELOAD are supported. See below for an example:
 
   - Example: ``{"OMP_NUM_THREADS": "32", "TF_WARNINGS": "none"}``
 
+  - Example: ``{"LD_LIBRARY_PATH": "${LD_LIBRARY_PATH}:/home/admin/my_lib"}``
+
 - ``container`` (dict): Require a given (Docker) image, and the worker process will run in a container with this image.
   The `worker_path` is the default_worker.py path. It is required only if ray installation directory in the container is different from raylet host.
   The `run_options` list spec is `here <https://docs.docker.com/engine/reference/run/>`__.