drm/atomic: Unconfuse the old_state mess in commmit_tail

I totally butcherd the job on typing the kernel-doc for these, and no
one realized. Noticed by Russell. Maarten has a more complete approach
to this confusion, by making it more explicit what the new/old state
is, instead of this magic switching behaviour.

v2:
- Liviu pointed out that wait_for_fences is even more magic. Leave
that as @state, and document @pre_swap better.
- While at it, patch in header for the reference section.
- Fix spelling issues Russell noticed.

v3: Fix up the @pre_swap note (Liviu): Also s/synchronous/blocking/,
since async flip is something else than non-blocking.

Cc: Liviu Dudau <liviu.dudau@arm.com>
Reported-by: Russell King - ARM Linux <linux@armlinux.org.uk>
Cc: Russell King - ARM Linux <linux@armlinux.org.uk>
Fixes: 9f2a7950e7 ("drm/atomic-helper: nonblocking commit support")
Cc: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Tomeu Vizoso <tomeu.vizoso@gmail.com>
Cc: Daniel Stone <daniels@collabora.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161121171802.24147-1-daniel.vetter@ffwll.ch

Documentation/gpu/drm-kms-helpers.rst

@@ -63,6 +63,9 @@ Atomic State Reset and Initialization
 .. kernel-doc:: drivers/gpu/drm/drm_atomic_helper.c
    :doc: atomic state reset and initialization
 
+Helper Functions Reference
+--------------------------
+
 .. kernel-doc:: include/drm/drm_atomic_helper.h
    :internal: