Document VC headers and other VC changes.

Document VC headers and other VC changes.

* maintaining.texi (Version Control): Say "commit", not "check in".
(Version Control Systems): Simplify descriptions.
(VCS Merging, VCS Changesets, VCS Repositories): New nodes, split from
VCS Concepts.
(VC Mode Line): Update example.
(Old Revisions): Document revert-buffer for vc-diff.
(Log Buffer): Promote to a subsection.  Document header lines.

* emacs.texi (Top): Update node listing.