Document the userspace ABI with kerneldoc to provide some information on
how to use it.

v3:
- reword description of arrays and array lengths

v2:
- keep GEM object creation flags for ABI compatibility
- fix typo in struct drm_tegra_syncpt_incr kerneldoc
- fix typos in struct drm_tegra_submit kerneldoc
- reworded some descriptions as suggested
Reviewed-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>