More updates for VC documentation.

More updates for VC documentation.

* doc/emacs/maintaining.texi (VCS Concepts): Make "revision" terminology
less CVS-specific.
(VC With A Merging VCS, VC With A Locking VCS): Add xref to
Registering node.
(Secondary VC Commands): Deleted.  Promote subnodes.
(Log Buffer): Add command name for C-c C-c.  Fix the name of the
log buffer.  Add index entries.
(VCS Changesets, Types of Log File, VC With A Merging VCS): Use
"commit" terminology.
(Old Revisions): Move it to just before VC Change Log.  "Tag" here
doesn't refer to tags tables.  Note other possible forms of the
revision ID.  C-x v = does not save.
(Registering): Note similarity to C-x v v action.  Fix description
of how backends are chosen.  De-document vc-default-init-revision.
(VC Change Log): Document C-x v l in VC-Dir buffer.  Document RET
in root log buffers.

* lisp/vc/vc.el (vc-deduce-fileset): Minor doc fix.