More updates for VC documentation.

More updates for VC documentation.

* doc/emacs/maintaining.texi (VCS Merging, VCS Changesets): Index entries.
(VC Mode Line): Add index entry for "version control status".
(VC Undo): Use vc-revert instead of its vc-revert-buffer alias.
Document vc-revert-show-diff.  De-document vc-rollback.
(VC Directory Mode): Rewrite introduction.  Move prefix arg
documentation here from VC Directory Buffer node.
(VC Directory Buffer): Use a decentralized VCS example.
(VC Directory Commands): Use a table.  Remove material duplicated
in previous nodes on multi-file VC filsets.