net: fix some documentation comments in UDPSocketPosix

They were saying that they set the flags in |socket_options_|,
but that is simply not true anymore.

BUG=None
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Review-Url: https://codereview.chromium.org/2819153002
Cr-Commit-Position: refs/heads/master@{#465557}
1 file changed