[xcbf] SIgnature verification text

I noted in ballot comments that we were missing text
describing signature verification. I'll add the following
text to the two places noted in the document after the CS
ballot closes.

Phil

For type DigitalSignature: 

To verify the signature in a digital signature integrity 
object, a hash is computed over the biometricObjects component
of type IntegrityObjects using the hash algorithm and any 
associated parameters indicated in the algorithmID component 
of type DigitalSignature. The resulting hash value is compared
to the value of the hash obtained from applying the signature
verification key to the signature component of type 
DigitalSignature to determine if this signature is valid.  

Similar text for type SignedData.