os: document that behavior of Seek on O_APPEND files is not specified

The actual behavior varies across platforms, and due to the inherent
race, we can't do anything better (other than to always return 0).

Fixes #12710.

Change-Id: Icb52f0f1f0a267e0f9f70767cae427f3f0239965
Reviewed-on: https://go-review.googlesource.com/14881Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/file.go

@@ -173,6 +173,7 @@ func (f *File) WriteAt(b []byte, off int64) (n int, err error) {
 // according to whence: 0 means relative to the origin of the file, 1 means
 // 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.
 func (f *File) Seek(offset int64, whence int) (ret int64, err error) {
 	if f == nil {
 		return 0, ErrInvalid