documentation: networking: can: Document alloc_candev_mqs()

documentation: networking: can: Document alloc_candev_mqs()

Since the introduction of alloc_candev_mqs() and friends, there is no
longer a need to allocate a generic network device and perform explicit
CAN-specific setup.  Remove the code showing this setup, and document
alloc_candev_mqs() instead.

Fixes: 39549eef3587f1c1 ("can: CAN Network device driver and Netlink interface")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://patch.msgid.link/c0f9a706ba31f1a49eb72e58526cd294d97a1ce9.1748865431.git.geert+renesas@glider.be
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>