drm/i915: Complete kerneldoc for struct i915_gem_context

The existing kerneldoc was outdated, so time for a refresh.

v2: Use single line kdoc, mention functions for manipulation

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161231112012.29263-3-chris@chris-wilson.co.uk