drm/i915: kerneldoc for i915_gem_shrinker.c

And remove one bogus * from i915_gem_gtt.c since that's not a
kerneldoc there.

v2: Review from Chris:
- Clarify memory space to better distinguish from address space.
- Add note that shrink doesn't guarantee the freed memory and that
  users must fall back to shrink_all.
- Explain how pinning ties in with eviction/shrinker.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>