media: v4l2- mediabus: Add usage note for V4L2_MBUS_*

With the removal of the legacy g_mbus_config and s_mbus_config video
operations, the sole users of V4L2_MBUS_* flags are now the newly
introduced get_mbus_config and set_mbus_config pad operations.

As the semantic of the new operations differs from the semantic of
the legacy ones, add a usage note in the v4l2-mediabus.h header to
specify how to use the flags.

Also add a TODO note to record that we intend to replace the existing
flags with fields, to prevent users from mixing conflicting values
in a single operation call.

Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>