drm/doc: Document ioctl errno value patterns

We're not super-consistent about these, but I think it's worth to
document at least the commmon patterns.

v2:
- Add a not about ENOTTY (it's just a confusing name, but used
exactly what it's meant for in DRM) (Chris).
- Unconfuse the text for ENODEV (Daniel)
- Move text undert the IOCTL heading (Chris).
- typos

Cc: Daniel Stone <daniel@fooishbar.org>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: "Zhang, Tina" <tina.zhang@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170818174328.6386-1-daniel.vetter@ffwll.ch