media: vb2-core: fix descriptions for VB2-only functions

When we split VB2 into an independent streaming module and
a V4L2 one, some vb2-core functions started to have a wrong
description: they're meant to be used only by the API-specific
parts of VB2, like vb2-v4l2, as the functions that V4L2 drivers
should use are all under videobuf2-v4l2.h.

Correct their descriptions.

Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>