media: v4l: uAPI doc: Changing frame interval won't change format

Document that changing the frame interval has no effect on frame size.
While this was the assumption in the API, it was not documented as such.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>