[3.11] GH-99334: Explain that PurePath.is_relative_to() is purely lexical. (GH-114031) (#114461)

(cherry picked from commit 3a61d24)

Author: barneygale

Doc/library/pathlib.rst

@@ -510,6 +510,13 @@ Pure paths provide the following methods and properties:
 
    If multiple arguments are supplied, they are joined together.
 
+   This method is string-based; it neither accesses the filesystem nor treats
+   "``..``" segments specially. The following code is equivalent:
+
+      >>> u = PurePath('/usr')
+      >>> u == p or u in p.parents
+      False
+
    .. versionadded:: 3.9