doc: remove use of "random port" regarding dgram send
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I removed the use of "random port" when referring to sending messages with dgram. Specifically in the examples the port isn't random, it is a hand-picked port which stays the same. Referring to it as a random port is confusing because in the listening methods the phrase "random port" has a special meaning, i.e. providing no port and letting the OS pick a random port.