doc: clarify safety of multiple and concurent os.(*File).Close() calls

Fixes #32427

Change-Id: I4b863bd3836067dcc2eb3a9c3a7169656763d003
Reviewed-on: https://go-review.googlesource.com/c/go/+/180438Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/os/file_plan9.go

@@ -136,6 +136,7 @@ func openFileNolog(name string, flag int, perm FileMode) (*File, error) {
 // Close closes the File, rendering it unusable for I/O.
 // On files that support SetDeadline, any pending I/O operations will
 // be canceled and return immediately with an error.
+// Close will return an error if it has already been called.
 func (f *File) Close() error {
 	if err := f.checkValid("close"); err != nil {
 		return err

src/os/file_unix.go

@@ -225,6 +225,7 @@ func openFileNolog(name string, flag int, perm FileMode) (*File, error) {
 // Close closes the File, rendering it unusable for I/O.
 // On files that support SetDeadline, any pending I/O operations will
 // be canceled and return immediately with an error.
+// Close will return an error if it has already been called.
 func (f *File) Close() error {
 	if f == nil {
 		return ErrInvalid

src/os/file_windows.go

@@ -178,6 +178,7 @@ func openFileNolog(name string, flag int, perm FileMode) (*File, error) {
 // Close closes the File, rendering it unusable for I/O.
 // On files that support SetDeadline, any pending I/O operations will
 // be canceled and return immediately with an error.
+// Close will return an error if it has already been called.
 func (file *File) Close() error {
 	if file == nil {
 		return ErrInvalid