Clarifying behavior of 'unique' option in HABTM

The docs here were unclear and - in the case of `false` - not even sensical, probably due to some errant edit history.  I've edited the copy to reflect the behavior as I understand it after reading the code and doing some tests to confirm.
Yikes, I had tried a few different wordings here, and apparently failed at editing.  Fixed these two, and re-read it a couple times to try to catch any other editing fails.