gh-77607: Improve accuracy of os.path.join docs (GH-101406)

This is a follow-up to GH-100811.

One of the changes in that PR isn't accurate in that
`os.path.join('', '')` will not end in a separator.

This reverts that change to the previous wording that used "only", but
explicitly calls out the case where the last part ends in a separator,
which is what caused confusin in GH-77607 and motivated the change
in GH-100811.
(cherry picked from commit 909a6746939ea1d09fab21f26b558cfd7e3e29a0)

Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>