Add return info to sha256 docs
Add return info to sha256 docs

Signed-off-by: Aaron Erhardt <2c31faf558b433b413d25a3a75f30963d83c923c@t-online.de>

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