can-doc: Add hint about getting timestamps

This patch adds a hint about how to get timestamps of received
CAN frames with ioctl(2). This hint has been applied to the
former SocketCAN Documentation, but it got lost during mainlining
the first bits and pieces to linux kernel.

Signed-off-by: Stefan Tatschner <rumpelsepp@sevenbyte.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>