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

This is a follow-up to #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 #77607 and motivated the change
in #100811.