Update Natspec for createContingentPool function

[Walodja1987]

The Natspec for the createContingentPool function says the following:
"expiryTime: Expiration time of the pool expressed as a unix timestamp in seconds (UTC). The value of the reference asset observed at that point in time determines the payoffs for long and short position tokens."

The second sentence is not entirely true for all situations. If the reference asset points to a file that specifies to report the value as of a date prior to expiryTime, then these specs need to be followed.