bpo-23082: Better error message for PurePath.relative_to() from pathlib (GH-19611)


Co-authored-by: Sadhana Srinivasan <rotuna@Sadhanas-MBP.fritz.box>
(cherry picked from commit 448325369ff73011d34d6c3a493014fe3ead8843)

Co-authored-by: Rotuna <sadhanasrinivasan@protonmail.com>
diff --git a/Doc/library/pathlib.rst b/Doc/library/pathlib.rst
index 83f7c83..bf6fee4 100644
--- a/Doc/library/pathlib.rst
+++ b/Doc/library/pathlib.rst
@@ -551,7 +551,9 @@
         File "<stdin>", line 1, in <module>
         File "pathlib.py", line 694, in relative_to
           .format(str(self), str(formatted)))
-      ValueError: '/etc/passwd' does not start with '/usr'
+      ValueError: '/etc/passwd' is not in the subpath of '/usr' OR one path is relative and the other absolute.
+
+   NOTE: This function is part of :class:`PurePath` and works with strings. It does not check or access the underlying file structure.
 
 
 .. method:: PurePath.with_name(name)