can: remove false statement about 1:1 mapping between DLC and length

can: remove false statement about 1:1 mapping between DLC and length

The CAN-FD section of can.rst still states that there is a 1:1 mapping
between the Classical CAN DLC and its length. This is only true for
the DLC values up to 8. Beyond that point, the length remains at 8.

For reference, the mapping between the CAN DLC and the length is given
in below table [1]:

	 DLC value	CBFF and CEFF	FBFF and FEFF
	 [decimal]	    [byte]	    [byte]
	----------------------------------------------
		 0		 0		 0
		 1		 1		 1
		 2		 2		 2
		 3		 3		 3
		 4		 4		 4
		 5		 5		 5
		 6		 6		 6
		 7		 7		 7
		 8		 8		 8
		 9		 8		12
		10		 8		16
		11		 8		20
		12		 8		24
		13		 8		32
		14		 8		48
		15		 8		64

Remove the erroneous statement. Instead just state that the length of
a Classical CAN frame ranges from 0 to 8.

[1] ISO 11898-1:2024, Table 5 -- DLC: coding of the four LSB

Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
Link: https://patch.msgid.link/20251013-can-fd-doc-v2-1-5d53bdc8f2ad@kernel.org
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>