[media] docs-rst: Make the CSI-2 bus initialisation documentation match reality

Update the CSI-2 bus documentation to tell that the LP-11 mode is not
mandatory as there are transmitters that cannot be explicitly set to LP-11
mode. Instead, say that this what the transmitter drivers shall do if
possible.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>