[3.15] gh-160322: document shutil.rmtree("link/") symlink following behavior (GH-160372) (#160393)

(cherry picked from commit 2c24bc1)

Co-authored-by: Gregory P. Smith <68491+gpshead@users.noreply.github.com>

Author: miss-islington

Doc/library/shutil.rst

@@ -313,6 +313,12 @@ Directory and files operations
    handled by calling a handler specified by *onexc* or *onerror* or, if both
    are omitted, exceptions are propagated to the caller.
 
+   If *path* is a symbolic link to a directory followed by a trailing path
+   separator, such as ``'link/'``, the operating system resolves the link, so
+   :func:`!rmtree` deletes the contents of the directory it points to and then
+   fails to remove *path* itself.  :class:`pathlib.Path` strips trailing
+   separators, so ``rmtree(Path('link/'))`` refuses the link as usual.
+
    This function can support :ref:`paths relative to directory descriptors
    <dir_fd>`.