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>

lib/igt_fb.c

@@ -544,6 +544,7 @@ cairo_t *igt_get_cairo_ctx(int fd, struct igt_fb *fb)
 }
 
 /**
+ * igt_write_fb_to_png:
  * @fd: open i915 drm file descriptor
  * @fb: pointer to an #igt_fb structure
  * @filename: target name for the png image