drm/doc: Improve ioctl/fops docs a bit more

drm/doc: Improve ioctl/fops docs a bit more

I spotted a markup issue, plus adding the descriptions in drm_driver.
Plus a few more links while at it.

I'm still mildly unhappy with the split between fops and ioctls, but I
still think having the ioctls in the uapi chapter makes more sense. Oh
well ...

v2: Rebase.

v3: Move misplace hunk to the right patch.

Cc: Stefan Agner <stefan@agner.ch>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170531092045.3950-1-daniel.vetter@ffwll.ch