emacs server variable server-name ... not available

[Harry Putnam]

[Note sorry if this is a double bubble... My initial post got away
from me before I had finished it... I attempted cancelling it but it
may not have worked... here is the complete message]

I've only been using emacsclient a short while.
I thought maybe I'd like to try having two servers.  One with gnus
running and another for some perl scripting I'm working on.

Reading parts of the documentation I see:
(info "(emacs)Emacs Server")
     You can run multiple Emacs servers on the same machine by giving
  each one a unique "server name", using the variable `server-name'.
  For example, `M-x set-variable <RET> server-name <RET> foo <RET>'
  sets the server name to `foo'.  The `emacsclient' program can
  specify a server by name, using the `-s' option (*note emacsclient
  Options::).
  
In a recent cvs of emacs-23, when I attempt to set that variable as
described I get a comment:  
  M-x set-variable <RET> server-name [no-match]

Emacs knows nothing about this variable... is there some particular
package that must be loaded for this variable to be recognized?