Rewording the comments for Join to do a better job of calling out
when Clean is called. Also clarifing other portions of the comment.

Fixes #29875

Change-Id: Ied43983bb10a97922898d28af133de0930224496
Reviewed-on: https://go-review.googlesource.com/c/go/+/194339Reviewed-by: Rob Pike <r@golang.org>