Add a section explaining the uses of PSA_WANT_xxx
Add a section explaining the uses of PSA_WANT_xxx

PSA_WANT_xxx is useful regardless of how the symbols are defined:
explicitly (with MBEDTLS_PSA_CRYPTO_CONFIG) or implicitly (without
MBEDTLS_PSA_CRYPTO_CONFIG).

Signed-off-by: Gilles Peskine <f805f64266d288fc5467baa7be6cd0ff366f477b@arm.com>

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