Clarify definition of resultIndex

I propose eliminating from the definition of resultIndex the sentence:
    "When continuous was false, the resultIndex must always be 0."

While this is true when returning the final result, this constraint is
already inherent in the spec because only a single final result may be
returned in continuous mode (5.1.1) and the results list must never contain
interim results before final results (5.1.8).

However, it is not necessarily true when returning interim results (for
example, when only some of the interim results need to be overwritten).

If there's no disagreement, I'll remove this sentence from the spec on
Tuesday.

/Glen Shires