drm/syncobj: Add better overview documentation for syncobj (v2)

This patch only brings the syncobj documentation up-to-date for the
original form of syncobj.  It does not contain any information about the
design of timeline syncobjs.

v2: Incorporate feedback from Lionel and Christian:
 - Mention actual ioctl and flag names
 - Better language around reference counting
 - Misc. language cleanups

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190812142211.15885-1-jason@jlekstrand.net