More updates for VC documentation.

More updates for VC documentation.

* doc/emacs/maintaining.texi (VCS Concepts): Add "working tree" terminology.
(Old Revisions): Use it.
(VCS Repositories): Add "distributed" terminology.
(Log Buffer): Remove duplicate description
about changesets.  Fix "current VC fileset" ambiguity.
(Multi-User Branching): Node deleted.
(Branches, Switching Branches): Discuss decentralized version control systems.
(VC Pull): New node.
(Merging): Document merging on decentralized systems.
(Creating Branches): Note that this is specific to CVS and related systems.