It is impossible to wait "at most" some time; the only theoretical guarantee is that the wait time will be no less than some number. This is in fact specified in the JLS. Therefore the wording must be read as follows:
Let t be the given time to wait, and let u be the time this method actually spent before returning control to the caller. If and only if u < t, the return value of the method call is guaranteed to be the return value of the Future.
Given the above, a negative t is fully in order and all values <= 0 should behave exactly the same.
Still, an implementation could conceivably throw an exception for a negative wait time and have a weak excuse for doing so. If your code ever failed due to such an exception, you could start a hot dispute with the authors of said Future
implementation. If that ever happened, there is really nothing an answer on Stack Overflow can do to prevent it.
If you subsequently went to court to settle the responsibility for damages, and are interested in the outcome of such a lawsuit, then I believe that concern is beyond the realms of Stack Overflow.