drm/sched: improve docs around drm_sched_entity

I found a few too many things that are tricky and not documented, so I
started typing.

I found a few more things that looked broken while typing, see the
varios FIXME in drm_sched_entity.

Also some of the usual logics:
- actually include sched_entity.c declarations, that was lost in the
  move here: 620e762f

 ("drm/scheduler: move entity handling into
  separate file")

- Ditch the kerneldoc for internal functions, keep the comments where
  they're describing more than what the function name already implies.

- Switch drm_sched_entity to inline docs.

Acked-by: Melissa Wen <mwen@igalia.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com> (v1)
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: Boris Brezillon <boris.brezillon@collabora.com>
Cc: Steven Price <steven.price@arm.com>
Cc: Emma Anholt <emma@anholt.net>
Cc: Lee Jones <lee.jones@linaro.org>
Cc: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210805104705.862416-7-daniel.vetter@ffwll.ch