Improve documentation of cn in x509_crt_verify()
Improve documentation of cn in x509_crt_verify()

Mention explicitly that only DNS names are supported so far, and while at it
explain where the name is searched.

Signed-off-by: Manuel Pégourié-Gonnard <4ba0d615aeeda88e422ca8fba28fe2938485facf@arm.com>

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