Add nxlinkConnectToHost note about the socket driver

[alloncm]

Add a comment that the socket driver must be initialized prior to calling this function, otherwise this function will fail.

[masagrator]

socket must be initialized for any kind of server-client interaction in network. That's not limited to this function, and that's not limited to this platform.

[alloncm]

You are correct I just assumed that it will be mentioned in the docs / have an indicative error message / the function will initialize it itself (though this might be risky on second thought).

Perhaps setting errno to an indicative error number would be better?

Anyway this is my first time developing for the switch so feel free to close this if you think it is trivial.