Clarify xcm_attr_set() on UTLS connection sockets

[m-ronnblom]

The UTLS transport documentation is clear on the fact that xcm_connect() or xcm_accept() results in either a UX or a TLS connection socket, but falsely claims that such connection sockets will accept "non-native" socket attribute (e.g., a UX connection socket will accept "tcp.user_timeout").

The only situation where both UX and TLS attributes, and potentially future UTLS-specif attributes, can be configured on a UTLS socket is at the point where the caller does not know, is at the time of XCM socket creation (e.g., xcm_connect_a()).