doc/go1.17: mention SYS_WAIT6/WEXITED on NetBSD
doc/go1.17: mention SYS_WAIT6/WEXITED on NetBSD

For #13987
For #16028
For #44513

Change-Id: I7a73446fcc80a01fa6de24eec1e5b993e543be37
Reviewed-on: https://go-review.googlesource.com/c/go/+/323489
Trust: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Emmanuel Odeke <bd234ba4276433f0e5fc7a8fa2d18274fa711567@orijtech.com>

CAFxX/go,nawawi/go,cloudflare/go,CAFxX/go,nawawi/go,nawawi/go,nawawi/go,nawawi/go,cloudflare/go,cloudflare/go,cloudflare/go,CAFxX/go,cloudflare/go,CAFxX/go,CAFxX/go,CAFxX/go,cloudflare/go,golang/go,nawawi/go,golang/go,CAFxX/go,golang/go,golang/go,nawawi/go,golang/go,CAFxX/go,golang/go,golang/go,golang/go,nawawi/go,cloudflare/go,cloudflare/go