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>