Update SAN parsing documentation
Update SAN parsing documentation

1) Fix typo in `mbedtls_x509_parse_subject_alt_name()` documentation.
2) Add a not in `mbedtls_x509_parse_subject_alt_name()` documentation,
stating that the lifetime of the target structure is restricted
by the lifetime ofthe parsed certificate.

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