Add comment describing the feature
Add comment describing the feature

Add a comment that describes that the feature only supportes the
`use_srtp` extension, and not hte full DTLS-SRTP RFC.

Signed-off-by: Johan Pascal <a9ed27abf9fd2327f3cdfbacd27d3e6c0500f5a6@belledonne-communications.com>

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