internal/poll: fix the inaccurate comment and add a edge test case
*BSD can also returns (>0, EAGAIN) with non-blocking socket,
it's therefore not macOS-specific.
Change-Id: I57a393da31fab7788ad5779a038396be8236b0f9
Reviewed-on: https://go-review.googlesource.com/c/go/+/623056
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Ian Lance Taylor <iant@google.com> Reviewed-by: Damien Neil <dneil@google.com>
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