drm/doc: Update docs about device instance setup

drm/doc: Update docs about device instance setup

->load is deprecated, bus functions are deprecated and everyone
should use drm_dev_alloc&register.

So update the .tmpl (and pull a bunch of the overview docs into the
sourcecode to increase chances that it'll stay in sync in the future)
and add notes to functions which are deprecated. I didn't bother to
clean up and document the unload sequence similarly since that one is
still a bit a mess: drm_dev_unregister does way too much,
drm_unplug_dev does what _unregister should be doing but then has the
complication of promising something it doesn't actually do (it doesn't
unplug existing open fds for instance, only prevents new ones).

Motivated since I don't want to hunt every new driver for usage of
drm_platform_init any more ;-)

v2: Reword the deprecation note for ->load a bit, using Laurent's
suggestion as an example (but making the wording a bit stronger even).
Fix spelling in commit message.

v3: More spelling fixes from Laurent.

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: David Herrmann <dh.herrmann@gmail.com>
Acked-by: David Herrmann <dh.herrmann@gmail.com> (v2)
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>