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>

src/syscall/exec_windows.go

@@ -247,7 +247,7 @@ type SysProcAttr struct {
 	Token                      Token               // if set, runs new process in the security context represented by the token
 	ProcessAttributes          *SecurityAttributes // if set, applies these security attributes as the descriptor for the new process
 	ThreadAttributes           *SecurityAttributes // if set, applies these security attributes as the descriptor for the main thread of the new process
-	NoInheritHandles           bool                // if set, each inheritable handle in the calling process is not inherited by the new process
+	NoInheritHandles           bool                // if set, no handles are inherited by the new process, not even the standard handles, contained in ProcAttr.Files, nor the ones contained in AdditionalInheritedHandles
 	AdditionalInheritedHandles []Handle            // a list of additional handles, already marked as inheritable, that will be inherited by the new process
 	ParentProcess              Handle              // if non-zero, the new process regards the process given by this handle as its parent process, and AdditionalInheritedHandles, if set, should exist in this parent process
 }