Clarified documentation of mbedtls_ssl_setup
Clarified documentation of mbedtls_ssl_setup

Note that the configuration structure must remain accessible. The
previous wording could have been taken as implying that it's ok to
change the structure but changes wouldn't be taken into account.

Also note that calling this function twice is not supported (it would
at least be a memory leak).

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