Improved error message when a purported rule does not exist.  Thanks to feedback from David Rager leading to this change.

git-svn-id: http://acl2-devel.googlecode.com/svn/trunk@1186 5d84a466-a78b-0930-a311-59b22171a541
(Mirroring acl2-books revision 2356) Added a paragraph to :doc for rule-classes explaining that not all events generate rules.  Thanks to feedback from David Rager leading to this change.

git-svn-id: http://acl2-devel.googlecode.com/svn/trunk@1187 5d84a466-a78b-0930-a311-59b22171a541