ecp_curves: Minor refactoring.

This patch introduces the following changes:
* Documentation for `mbedtls_ecp_modulus_setup()`
  moved to `ecp_invasive.h`.
* Added invalid modulus selector `MBEDTLS_ECP_MOD_NONE`.
* Adjusted negative tests to use invalid selectors.
* Reworded documentation.

Signed-off-by: Minos Galanakis <minos.galanakis@arm.com>

library/ecp_invasive.h

@@ -118,6 +118,23 @@ int mbedtls_ecp_mod_p521_raw(mbedtls_mpi_uint *X, size_t X_limbs);
 
 #endif /* MBEDTLS_ECP_DP_SECP521R1_ENABLED */
 
+/** Initialise a modulus with hard-coded const curve data.
+ *
+ * \note            The caller is responsible for the \p N modulus' memory.
+ *                  mbedtls_mpi_mod_modulus_free(&N) should be invoked at the
+ *                  end of its lifecycle.
+ *
+ * \param[in,out] N The address of the modulus structure to populate.
+ *                  Must be initialized.
+ * \param[in] id    The mbedtls_ecp_group_id for which to initialise the modulus.
+ * \param[in] ctype The mbedtls_ecp_curve_type identifier for a coordinate modulus (P)
+ *                  or a scalar modulus (N).
+ *
+ * \return          \c 0 if successful.
+ * \return          #MBEDTLS_ERR_ECP_BAD_INPUT_DATA if the given MPIs do not
+ *                  have the correct number of limbs.
+ *
+ */
 MBEDTLS_STATIC_TESTABLE
 int mbedtls_ecp_modulus_setup(mbedtls_mpi_mod_modulus *N,
                               const mbedtls_ecp_group_id id,