os: document that File.Seek works on directories, but not portably

Updates #36019

Change-Id: I9fea2c3c5138e2233290979e4724f6e7b91da652
Reviewed-on: https://go-review.googlesource.com/c/go/+/213439Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/os/file.go

@@ -204,6 +204,10 @@ func (f *File) WriteAt(b []byte, off int64) (n int, err error) {
 // relative to the current offset, and 2 means relative to the end.
 // It returns the new offset and an error, if any.
 // The behavior of Seek on a file opened with O_APPEND is not specified.
+//
+// If f is a directory, the behavior of Seek varies by operating
+// system; you can seek to the beginning of the directory on Unix-like
+// operating systems, but not on Windows.
 func (f *File) Seek(offset int64, whence int) (ret int64, err error) {
 	if err := f.checkValid("seek"); err != nil {
 		return 0, err