Improved GetThresholdKeyGenerator documentation

Added information about the required minimum bit length of the public
key. Removed the threshold information as we are not sure now if it's
valid. We'll investigate it separately and make sure a proper validation
for threshold is made.

thresholdkey_generator.go

@@ -46,10 +46,11 @@ type ThresholdKeyGenerator struct {
 	polynomialCoefficients []*big.Int
 }
 
-// Preferable way to construct the ThresholdKeyGenerator. No verification
-// is done on the input values.  You need to be sure that nbits is big enough
-// and that Threshold > TotalNumberOfDecryptionServers / 2.
-// The plaintext space for the key will be Z_n.
+// GetThresholdKeyGenerator is a preferable way to construct the
+// ThresholdKeyGenerator.
+// Due to the various properties that must be met for the threshold key to be
+// considered valid, the minimum public key `N` bit length is 18 bits.
+// The plaintext space for the key will be `Z_N`.
 func GetThresholdKeyGenerator(
 	publicKeyBitLength int,
 	totalNumberOfDecryptionServers int,