PROB: check and specify details

Broken out from OFFICE-1170

PROB
*This needs to specify if Data must be ordered or not (unordered?).
*It would also be better to be clear about whether Data must be sequential (is 1, 2, 3, 5 (missing 4) OK?) and/or integer (is 1, 2.5, 3,4 OK?).
*It would also be better to be clear about whether Start and End need to exist within Data (no?) and also whether they can be higher/lower than the max/min values in Data (yes?)

• It would also be better to be clear about whether Start can be higher than End. Although they're defined as lower/upper bounds, is 'Start>End' an error, or valid with a probability of 0 returned?
• This needs to specify how the result is calculated if End is omitted and Start is not present in Data (Calc returns 0).
• This needs to specify the type of error returned - eg #N/A if Data Probability are different lengths. This is important because #N/A is a specific (testable) type of error.