media: Documentation: Update documentation for streams

Document how streams interacts with formats and selections.

Update documentation in respect to what is allowed, in particular streams
are only supported via full routes, source-only routes are not supported
right now.

The centerpiece of the API additions are streams. Albeit routes are
configured via S_ROUTING IOCTL that also declares streams, it is streams
that are accessed through other APIs. Thus refer to streams instead of
routes in documentation.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>