Add output initialization requirement
Signed-off-by: Werner Lewis <werner.lewis@arm.com>
diff --git a/library/bignum_mod.h b/library/bignum_mod.h
index ee5e0bf..e03c9aa 100644
--- a/library/bignum_mod.h
+++ b/library/bignum_mod.h
@@ -19,7 +19,9 @@
* 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