Backport phpdoc improvements from D6 core

As per http://drupal.org/node/155483#comment-284860 it'd be nice to backport all the phpdoc goodness I added to this module when I was getting it ready for D6 core. Almost all of it could just be cut and pasted directly from the core codebase, if someone wanted to do something useful that didn't require much serious thought or coding skills... The only difference is that in core, everything is "update_foo" whereas here, it's all "update_status_foo", since we renamed the module to better match core's existing conventions.