[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>

Documentation/media/kapi/csi2.rst

@@ -45,10 +45,11 @@ where
    * - bits_per_sample
      - Number of bits per sample.
 
-The transmitter drivers must configure the CSI-2 transmitter to *LP-11
-mode* whenever the transmitter is powered on but not active. Some
-transmitters do this automatically but some have to be explicitly
-programmed to do so.
+The transmitter drivers must, if possible, configure the CSI-2
+transmitter to *LP-11 mode* whenever the transmitter is powered on but
+not active. Some transmitters do this automatically but some have to
+be explicitly programmed to do so, and some are unable to do so
+altogether due to hardware constraints.
 
 Receiver drivers
 ----------------