Fixed minor :doc bugs (mostly pointed out by Jared).

Improved :doc disable-forcing.

Added a comment to :doc note-6-4 about the recent time$ improvement
that I'd forgotten to add previously.


git-svn-id: http://acl2-devel.googlecode.com/svn/trunk@1120 5d84a466-a78b-0930-a311-59b22171a541