drm/syncobj: Add documentation for timeline syncobj

drm/syncobj: Add documentation for timeline syncobj

We've added a set of new APIs to manipulate syncobjs holding timelines
of dma_fence. This adds a bit of documentation about how this works.

v2: Small language nits (Lionel)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/348578/
Cc: Christian Koenig <Christian.Koenig@amd.com>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Cc: David(ChunMing) Zhou <David1.Zhou@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>