Add output initialization requirement
* modified by the function.
* - Bignum parameters called \c X, \c Y, ... are outputs or input-output.
* The initial bignum value of output-only parameters is ignored, but
- * they must be set up and associated with the modulus \c N.
+ * they must be set up and associated with the modulus \c N. Some
+ * functions (typically constant-flow) require that the limbs in an
+ * output residue are initialized.
* - Bignum parameters called \c P are inputs used to set up a modulus or
* residue. These must be pointers to an array of limbs.
* - \c T is a temporary storage area. The initial content of such a
