media: docs-rst: media: Document broken frame handling in stream stop for CSI-2

Some CSI-2 transmitters will finish an ongoing frame whereas others will
not. Document that receiver drivers should not assume a particular
behaviour but to work in both cases.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>