gh-87691: clarify use of anchor in pathlib docs (#100782)

This is feedback from https://github.com/python/cpython/pull/100737#discussion_r1062968696

This matches the wording from the `os.path.join` docs better:
https://docs.python.org/3/library/os.path.html#os.path.join

In particular, the previous use of "anchor" was incorrect given the
pathlib definition of "anchor".

Co-authored-by: Barney Gale <barney.gale@gmail.com>