Add comment on how mbedtls_gcm_context::buf data depends on values of add_len and len.
Add comment on how mbedtls_gcm_context::buf data depends on
values of add_len and len.

Signed-off-by: Mateusz Starzyk <852a08ab448cfc7b533364fd73d7da3cb43a4d1d@mobica.com>

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