drm: Update TTM initialization documentation

ttm_global_reference was renamed to drm_global_reference.  This updates
the documentation to reflect that.  While we are there, document the
drm_global_reference API and update the initialization interface
documentation.

Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
[danvet: Keep the warning, ttm docs are still massively inadequate.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161228143216.26821-7-krisman@collabora.co.uk