os/exec: document ExtraFiles is not supported on windows

Fixes #26182

Change-Id: I1181e191f4742f166c9b67a6f41332a237cf0ede
Reviewed-on: https://go-review.googlesource.com/123855
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: adamdecaf

src/os/exec/exec.go

@@ -113,6 +113,8 @@ type Cmd struct {
 	// ExtraFiles specifies additional open files to be inherited by the
 	// new process. It does not include standard input, standard output, or
 	// standard error. If non-nil, entry i becomes file descriptor 3+i.
+	//
+	// ExtraFiles is not supported on Windows.
 	ExtraFiles []*os.File
 
 	// SysProcAttr holds optional, operating system-specific attributes.