can: add a note that RECV_OWN_MSGS frames are subject to filtering

Some parts of the documentation may lead the reader to think that the
socket's own frames are always received when CAN_RAW_RECV_OWN_MSGS is
enabled, but all frames are subject to filtering.

As explained by Marc Kleine-Budde:

On TX complete of a CAN frame it's pushed into the RX path of the
networking stack, along with the information of the originating socket.

Then the CAN frame is delivered into AF_CAN, where it is passed on to
all registered receivers depending on filters. One receiver is the
sending socket in CAN_RAW. Then in CAN_RAW the it is checked if the
sending socket has RECV_OWN_MSGS enabled.

Link: https://lore.kernel.org/r/20210420191212.42753-1-erik@flodin.me


Signed-off-by: Erik Flodin <erik@flodin.me>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>