Document that curve lists can include partially-supported curves
Document that curve lists can include partially-supported curves

Document that a curve returned by mbedtls_ecp_curve_list() or
mbedtls_ecp_grp_id_list() may lack support for ECDH or ECDSA.

Add a corresponding changelog entry, under "API Changes" because we
have changed the behavior: formerly, these functions skipped ECDH-only
curves, although this was not documented.

NXPmicro/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls