bpo-22021: Update root_dir and base_dir documentation in shutil (GH-10367)

Also added an example in shutil in order to make more clear how they are to be used.

Initially reported by Weinan Li on bpo.
(cherry picked from commit 7633371dac)

Co-authored-by: Lysandros Nikolaou <lisandrosnik@gmail.com>

Doc/library/shutil.rst

@@ -567,12 +567,14 @@ provided.  They rely on the :mod:`zipfile` and :mod:`tarfile` modules.
    available), or "xztar" (if the :mod:`lzma` module is available).
 
    *root_dir* is a directory that will be the root directory of the
-   archive; for example, we typically chdir into *root_dir* before creating the
-   archive.
+   archive, all paths in the archive will be relative to it; for example,
+   we typically chdir into *root_dir* before creating the archive.
 
    *base_dir* is the directory where we start archiving from;
    i.e. *base_dir* will be the common prefix of all files and
-   directories in the archive.
+   directories in the archive.  *base_dir* must be given relative
+   to *root_dir*.  See :ref:`shutil-archiving-example-with-basedir` for how to
+   use *base_dir* and *root_dir* together.
 
    *root_dir* and *base_dir* both default to the current directory.
 
@@ -724,6 +726,48 @@ The resulting archive contains:
     -rw-r--r-- tarek/staff   37192 2010-02-06 18:23:10 ./known_hosts
 
 
+.. _shutil-archiving-example-with-basedir:
+
+Archiving example with *base_dir*
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+In this example, similar to the `one above <shutil-archiving-example_>`_,
+we show how to use :func:`make_archive`, but this time with the usage of
+*base_dir*.  We now have the following directory structure:
+
+.. code-block:: shell-session
+
+    $ tree tmp
+    tmp
+    └── root
+        └── structure
+            ├── content
+                └── please_add.txt
+            └── do_not_add.txt
+
+In the final archive, :file:`please_add.txt` should be included, but
+:file:`do_not_add.txt` should not.  Therefore we use the following::
+
+    >>> from shutil import make_archive
+    >>> import os
+    >>> archive_name = os.path.expanduser(os.path.join('~', 'myarchive'))
+    >>> make_archive(
+    ...     archive_name,
+    ...     'tar',
+    ...     root_dir='tmp/root',
+    ...     base_dir='structure/content',
+    ... )
+    '/Users/tarek/my_archive.tar'
+
+Listing the files in the resulting archive gives us:
+
+.. code-block:: shell-session
+
+    $ python -m tarfile -l /Users/tarek/myarchive.tar
+    structure/content/
+    structure/content/please_add.txt
+
+
 Querying the size of the output terminal
 ----------------------------------------