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>

include/mbedtls/ecp.h

@@ -142,10 +142,11 @@ typedef enum {
 } mbedtls_ecp_curve_type;
 
 /*
- * Curve moduli types
+ * Curve modulus types
  */
 typedef enum {
-    MBEDTLS_ECP_MOD_COORDINATE = 0,
+    MBEDTLS_ECP_MOD_NONE = 0,
+    MBEDTLS_ECP_MOD_COORDINATE,
     MBEDTLS_ECP_MOD_SCALAR
 } mbedtls_ecp_modulus_type;