COBranch: document -undo and -redo. In -undo, disable the check that was preventing you from stepping back in a cheap copy to before the copy was made (or in a branch to before the branch was created)

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@10185 7b3f36aa-e6db-49c1-8b4c-66eac1997e64