Fix documentation for block size getters
Fix documentation for block size getters

- Document unit (bytes)
- Explain what happens for stream ciphers

Signed-off-by: Max Fillinger <0706025b2bbcec1ed8d64822f4eccd96314938d0@max-fillinger.net>

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