gh-132416: document behavior of os.symlink when dst already exists

Author: alex-semenyuk

Doc/library/os.rst

@@ -3510,6 +3510,8 @@ features:
    as a directory if *target_is_directory* is ``True`` or a file symlink (the
    default) otherwise.  On non-Windows platforms, *target_is_directory* is ignored.
 
+   If the *dst* already exists, :exc:`FileExistsError` is raised.
+
    This function can support :ref:`paths relative to directory descriptors
    <dir_fd>`.