Correct usage of key_derivation_output_key
Correct usage of key_derivation_output_key

Add the mention of key_derivation_output_key in the section about the
key derivation entry point family.

Rename "derive_key" to "key_derivation_output_key". At this point,
there's no reason to deviate from the naming convention.

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

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