Clarification on usage of nbf claim

[sloops77]

There is still some residual confusion around the use of nbf that should be clarified.

The spec currently mentions that

     When the iat and/or exp JWT claims are present, they represent the issuance and expiration 
     time of the signature, respectively. Note that these are different from the validFrom and 
     validUntil properties defined in Validity Period, which represent the validity of the data that 
     is being secured.

I think nbf should be mentioned in this paragraph as well, as the same interoperability concern exists here as there may be some discrepancies between how verifiers handle nbf vs how validFrom is treated.