Document parts not covered by USE_PSA_CRYPTO
Document parts not covered by USE_PSA_CRYPTO

Also, remove the section about design considerations for now. It's
probably more suitable for a developer-oriented document that would also
include considerations about possible paths for the future, which would
better be separated from user documentation (separating the certain that
is now, from the uncertain that might or might not be later).

Signed-off-by: Manuel Pégourié-Gonnard <4ba0d615aeeda88e422ca8fba28fe2938485facf@arm.com>

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