net: update the doc for TCPConn.SetKeepAlivePeriod on Windows
The method comment of TCPConn.SetKeepAlivePeriod had become
obsolete and inaccurate since CL 565495 and CL 570077 were merged.
For #65817
Change-Id: Ide99b2949676d452a505ba6fd634088f05c9df44
Reviewed-on: https://go-review.googlesource.com/c/go/+/576435 Reviewed-by: Than McIntosh <thanm@google.com> Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
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