Tidy up Context2D transform-related documentation.

Also adds back some images that
61d35829eed6661c1371d397bbb316ea2f636c50 removed
(4a5d31a56dfd871f3f7c5ce162885ebc870b8803 specified the wrong paths to
all but one file, which is probably why they were cleaned up).

Change-Id: Idd42bb831c97935e53ce05c69ea2691a2769f453
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>