Add comments explaining non-standard polynomial

The polynomial computed adds an extra x^k term which doesn't affect security, but could make the code more difficult to understand to someone comparing the code to wikipedia. Comment the divergence in both places where it occurs.

ssss.c

@@ -338,8 +338,12 @@ void encode_mpz(mpz_t x, enum encdec encdecmode)
   assert(mpz_sizeinbits(x) <= degree);
 }
 
-/* evaluate polynomials efficiently */
-
+/* evaluate polynomials efficiently
+ * Note that this implementation adds an additional x^k term. This term is 
+ * subtracted off on recombining. This additional term neither adds nor removes 
+ * security but is left solely for legacy reasons.
+ */
+
 void horner(int n, mpz_t y, const mpz_t x, const mpz_t coeff[])
 {
   int i;
@@ -522,6 +526,7 @@ void combine(void)
     }
     mpz_init(y[i]);
     field_import(y[i], b, 1);
+    /* Remove x^k term. See comment at top of horner() */
     field_mult(x, x, A[0][i]);
     field_add(y[i], y[i], x);
   }