Improve documentation of VC commands, including in Dired

Improve documentation of VC commands, including in Dired

* doc/emacs/dired.texi (Operating on Files):
* doc/emacs/maintaining.texi (VC Change Log, VC Directory Mode)
(Version Control, Basic VC Editing): Document VC command
invocation from Dired buffers.  Improve documentation of vc-log
commands.

* lisp/dired-aux.el (dired-vc-next-action):
* lisp/vc/vc.el (vc-print-log, vc-log-search, vc-log-mergebase)
(vc-log-view-type, vc-print-root-log, vc-next-action): Doc fixes.