vc docstrings

[Miles Bader]

I notice `vc-next-action' has a docstring of the form:

   SHORT_DESCRIPTION

   For locking systems:
   ... LONG DESCRIPTION FOR LOCKING VCS

   For merging systems:
   ... LONG DESCRIPTION FOR MERGING VCS

Given that all commonly used source-control systems these days are
the "merging" type, shouldn't the section on "merging systems" be first?

Also, I think many users are probably only vaguely aware that there _is_
such a distinction (how many people these days have ever used RCS?), so
perhaps it would be a good idea to give a list of common systems of each
type directly in the comment.

E.g., change the "merging systems" header to:

   For merging systems (CVS, Subversion, Git, Bzr, Hg, Arch):

[Maybe similar comments apply to other vc-... commands too.]

Thanks,

-Miles

-- 
"Most attacks seem to take place at night, during a rainstorm, uphill,
 where four map sheets join."   -- Anon. British Officer in WW I