lib/igt_core: Document testrunner interface a bit

Also fix up one gtkdoc fumble in igt_fb. We should use symbolic
defines if possible instead of just listening the magic 0, 77, 78
values for exit codes, but that's a separate patch.

Cc: tim.gore@intel.com
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
2 files changed