For me, the ambiguity is due to the phrases "exactly one directory separator" and "the only way to produce a trailing separator".

I would suggest:

"The return value is the concatenation of path and any members of *paths so that there is a directory separator (os.sep) following each part except the last. An empty part is ignored unless it is the last part, in which case the result will end in a separator."

Or:

"The return value is the concatenation of path and any members of *paths such that there is guaranteed to be a directory separator (os.sep) following each part except the last. An empty part is ignored unless it is the last part, in which case the result will end in a separator."