Add docs for mbedtls_ecp_mod_p448()

Signed-off-by: Paul Elliott <paul.elliott@arm.com>

library/ecp_invasive.h

@@ -243,6 +243,25 @@ int mbedtls_ecp_mod_p256k1_raw(mbedtls_mpi_uint *X, size_t X_limbs);
 
 #if defined(MBEDTLS_ECP_DP_CURVE448_ENABLED)
 
+/** Fast quasi-reduction modulo p448 = 2^448 - 2^224 - 1
+ * Write X as A0 + 2^448 A1 and A1 as B0 + 2^224 B1, and return A0 + A1 + B1 +
+ * (B0 + B1) * 2^224.
+ *
+ * \param[in,out]   X       The address of the MPI to be converted.
+ *                          Must have exact limb size that stores a 896-bit MPI
+ *                          (double the bitlength of the modulus). Upon return
+ *                          holds the reduced value which is in range `0 <= X <
+ *                          N` (where N is the modulus). The bitlength of the
+ *                          reduced value is the same as that of the modulus
+ *                          (448 bits).
+ * \param[in]       X_limbs The length of \p X in limbs.
+ *
+ * \return          \c 0 on Success.
+ * \return          #MBEDTLS_ERR_ECP_BAD_INPUT_DATA if \p X does not have
+ *                  twice as many limbs as the modulus.
+ * \return          #MBEDTLS_ERR_ECP_ALLOC_FAILED if memory allocation
+ *                  failed.
+ */
 MBEDTLS_STATIC_TESTABLE
 int mbedtls_ecp_mod_p448(mbedtls_mpi_uint *X, size_t X_limbs);