docs: networking: can: change bcm_msg_head frames member to support flexible array

The documentation of the 'bcm_msg_head' struct does not match how
it is defined in 'bcm.h'. Changed the frames member to a flexible array,
matching the definition in the header file.

See commit 94dfc73 ("treewide: uapi: Replace zero-length arrays with
flexible-array members")

Signed-off-by: Alex Tran <alex.t.tran@gmail.com>
Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>
Link: https://patch.msgid.link/20250904031709.1426895-1-alex.t.tran@gmail.com
Fixes: 94dfc73 ("treewide: uapi: Replace zero-length arrays with flexible-array members")
Link: https://bugzilla.kernel.org/show_bug.cgi?id=217783
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

Author: alectric-tr

Documentation/networking/can.rst

@@ -742,7 +742,7 @@ The broadcast manager sends responses to user space in the same form:
             struct timeval ival1, ival2;    /* count and subsequent interval */
             canid_t can_id;                 /* unique can_id for task */
             __u32 nframes;                  /* number of can_frames following */
-            struct can_frame frames[0];
+            struct can_frame frames[];
     };
 
 The aligned payload 'frames' uses the same basic CAN frame structure defined