pkgs/misc/misc.nix: removed the warning I added earlier

What I did was to add two versions of the same package into the same
collection, i.e. ghc-7.0.4 and ghc-7.2.1. I was surprised to see the
build succeed, because there are file collisions between these two. So I
(wrongly) assumed that the collection function was to blame, but in fact
this is a "feature" of nix-env. Apparently, file collision detection
doesn't take place when the user installs two versions of the same
package simultaneously. File collisions are detected only between
different packages!

For example, the command

  nix-env -i ghc-7.0.4 ghc-7.2.1-wrapper

is going to fail, but

  nix-env -i ghc-7.0.4-wrapper ghc-7.2.1-wrapper

is going to succeed. Maybe I just didn't read the documentation
thoroughly enough, but this behavior sure is unexpected to me.

svn path=/nixpkgs/trunk/; revision=28638

pkgs/misc/misc.nix

@@ -16,10 +16,6 @@ in
 	    [ pkgs.ruby l.chronic l.sup ];
 	};
       }
-
-    Warning: Using this function, it is possible to install packages
-    into one profile that have file collisions, and these will not be
-    detected.
   */
   collection = {list, name} : runCommand "collection-${name}" {} ''
     mkdir -p $out/nix-support