No doc string for system-type

[Nick Roberts]

Currently `C-h v sytem-type' doesn't display the documentation string:

> system-type's value is gnu/linux
> 
> Not documented as a variable.

This looks like its because its been cut and pasted from a patch:

  DEFVAR_LISP ("system-type", &Vsystem_type,
+              doc: /* Value is symbol indicating type of operating system you 
are using.
+Special values:
+  `gnu/linux'   compiled for a GNU/Linux system.
+  `darwin'      compiled for Darwin (GNU-Darwin, Mac OS X, ...).
+  `macos'       compiled for Mac OS 9.
+  `ms-dos'      compiled as an MS-DOS application.
+  `windows-nt'  compiled as a native W32 application.
+  `cygwin'      compiled using the Cygwin library.
+  `vax-vms' or `axp-vms': compiled for a (Open)VMS system.
+Anything else indicates some sort of Unix system.  */);

(this is the actual code not part of my patch).

It looks like an obvious error but I haven't committed a change in case there
is a reason e.g legal, ideological, for not having a doc string.

Nick