Fix documentation

Signed-off-by: Gabor Mezei <gabor.mezei@arm.com>
diff --git a/library/bignum_core.h b/library/bignum_core.h
index 5750656..7a0311a 100644
--- a/library/bignum_core.h
+++ b/library/bignum_core.h
@@ -401,6 +401,7 @@
 /**
  * \brief Perform a known-size multiplication
  *
+ * \p X may not be aliased to any of the inputs for this function.
  * \p A may be aliased to \p B.
  *
  * \param[out] X     The pointer to the (little-endian) array to receive