Improve docs for mbedtls_mpi_core_cond_assign
Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/library/bignum_core.h b/library/bignum_core.h
index 64d51b9..e5500f1 100644
--- a/library/bignum_core.h
+++ b/library/bignum_core.h
@@ -178,7 +178,9 @@
* \param[in] A The address of the source MPI. This must be initialized.
* \param limbs The number of limbs of \p A.
* \param assign The condition deciding whether to perform the
- * assignment or not.
+ * assignment or not. Callers will need to use
+ * the constant time interface (e.g. `mbedtls_ct_bool()`)
+ * to construct this argument.
*
* \note This function avoids leaking any information about whether
* the assignment was done or not.