[Doc] [runtime env] Add note that excludes paths are relative to wo…

…rking_dir (ray-project#25874)

Users' intuition might lead them to fill out `excludes` with absolute paths, e.g. `/Users/working_dir/subdir/`.  However, the `excludes` field uses `gitignore` syntax.  In `gitignore` syntax, paths that start with `/` are interpreted relative to the level of the directory where the `gitignore` file resides, and in our case this is the `working_dir` directory (morally speaking, since there's no actual `.gitignore` file.)  So the correct thing to put in `excludes` would be `/subdir/`.  As long as we support `gitignore` syntax, we should have a note in the docs for this.  This PR adds the note.

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

@@ -314,9 +314,11 @@ The ``runtime_env`` is a Python dictionary or a python class :class:`ray.runtime
   Note: This feature is currently limited to modules that are packages with a single directory containing an ``__init__.py`` file.  For single-file modules, you may use ``working_dir``.
 
 - ``excludes`` (List[str]): When used with ``working_dir`` or ``py_modules``, specifies a list of files or paths to exclude from being uploaded to the cluster.
-  This field also supports the pattern-matching syntax used by ``.gitignore`` files: see `<https://git-scm.com/docs/gitignore>`_ for details.
+  This field uses the pattern-matching syntax used by ``.gitignore`` files: see `<https://git-scm.com/docs/gitignore>`_ for details.
+  Note: In accordance with ``.gitignore`` syntax, if there is a separator (``/``) at the beginning or middle (or both) of the pattern, then the pattern is interpreted relative to the level of the ``working_dir``.
+  In particular, you shouldn't use absolute paths (e.g. `/Users/my_working_dir/subdir/`) with `excludes`; rather, you should use the relative path `/subdir/` (written here with a leading `/` to match only the top-level `subdir` directory, rather than all directories named `subdir` at all levels.)
 
-  - Example: ``["my_file.txt", "path/to/dir", "*.log"]``
+  - Example: ``{"working_dir": "/Users/my_working_dir/", "excludes": ["my_file.txt", "/subdir/, "path/to/dir", "*.log"]}``
 
 - ``pip`` (dict | List[str] | str): Either (1) a list of pip `requirements specifiers <https://pip.pypa.io/en/stable/cli/pip_install/#requirement-specifiers>`_, (2) a string containing the path to a pip
   `“requirements.txt” <https://pip.pypa.io/en/stable/user_guide/#requirements-files>`_ file, or (3) a python dictionary that has three fields: (a) ``packages`` (required, List[str]): a list of pip packages,