Block module docs cleanup

branching from #674474: Review and update D7 core module handbook pages - i want to do some cleaning up of the block module docs http://drupal.org/documentation/modules/block to make it more consistent with #1142004: Create Module Docs landing page standard

in particular http://drupal.org/documentation/modules/block should probably have http://drupal.org/getting-started/6/admin/build/block merged into it, and cleaned up.