media: docs-rst: Clarify duration of LP-11 mode

Add a sentence that makes it more clear when the CSI-2 transmitter
must, if possible, exit LP-11 mode. That is, maintain LP-11 mode
until stream on, at which point the transmitter activates the clock
lane and transition to HS mode.

Signed-off-by: Steve Longerbeam <slongerbeam@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>