[3.11] GH-101130: Document multiple arguments for PurePath.[is_]relative_to() (#114034)

3.11-only change as this is already covered in the 3.12+ deprecation
warning.

Author: barneygale

Doc/library/pathlib.rst

@@ -508,6 +508,8 @@ Pure paths provide the following methods and properties:
       >>> p.is_relative_to('/usr')
       False
 
+   If multiple arguments are supplied, they are joined together.
+
    .. versionadded:: 3.9
 
 
@@ -589,6 +591,8 @@ Pure paths provide the following methods and properties:
           .format(str(self), str(formatted)))
       ValueError: '/etc/passwd' is not in the subpath of '/usr' OR one path is relative and the other absolute.
 
+   If multiple arguments are supplied, they are joined together.
+
    NOTE: This function is part of :class:`PurePath` and works with strings. It does not check or access the underlying file structure.