Revert "intel: Fix documentation for drm_intel_gem_bo_wait()"

This reverts commit 080b4929.

Chris noticed that "negative values wait forever" is indeed intended
behaviour and the issue is just that we didn't have a testcase (fixed
now) and that a regression slipped through (fixed and on track for all
stable kernels).

So lets undo the documentation change for consistency, since working
around kernel regressions isn't good. Practical impact is nil anyway.

v2: Add a note to docs that some kernels have been broken.

v3: Remove the random garbage included by accident.

Cc: Kristian Høgsberg <krh@bitplanet.net>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>