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.
Good catch, I fixed this and another nearby instance that my edits didn't touch.  Didn't see any other instances in this file.