diff --git a/library/bignum_mod_raw.h b/library/bignum_mod_raw.h
index bcb4a1596c..56a9f860d6 100644
--- a/library/bignum_mod_raw.h
+++ b/library/bignum_mod_raw.h
@@ -158,15 +158,19 @@ int mbedtls_mpi_mod_raw_write( const mbedtls_mpi_uint *A,
 /**
  * \brief Perform a known-size modular addition.
  *
- * Calculate `A + B modulo N` where \p A, \p B, and \p N have the same size.
+ * Calculate `A + B modulo N`.
+ *
+ * The number of limbs in each operand, and the result, is given by the
+ * modulus \p N.
+ *
+ * \p X may be aliased to \p A or \p B, or even both, but may not overlap
+ * either otherwise.
  *
  * \param[out] X    The result of the modular addition.
  * \param[in] A     Little-endian presentation of the left operand. This
- *                  must be smaller than \p N, and have the same number of
- *                  limbs.
+ *                  must be smaller than \p N.
  * \param[in] B     Little-endian presentation of the right operand. This
- *                  must be smaller than \p N, and have the same number of
- *                  limbs.
+ *                  must be smaller than \p N.
  * \param[in] N     The address of the modulus.
  */
 void mbedtls_mpi_mod_raw_add( mbedtls_mpi_uint *X,