syscall: clarify which handles are affected by SysProcAttr.NoInheritHandles

syscall: clarify which handles are affected by SysProcAttr.NoInheritHandles

SysProcAttr.NoInheritHandles doc comment is not clear about which
handles are affected by it. This CL clarifies that it not only affects
the ones passed in AdditionalInheritedHandles, but also the ones
passed in ProcAttr.Files, which are required to be stderr, stdin and
stdout when calling syscall.StartProcess.

Updates #60942

Change-Id: I5bc5b3604b6db04b83f6764d5c5ffbdafeeb22fb
Reviewed-on: https://go-review.googlesource.com/c/go/+/505515
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>