Modernise the GRC formula somewhat

This was one of the first ones I wrote, so it lacks some of our more recent touches.

Library/Homebrew/formula.rb

@@ -96,8 +96,9 @@ class Formula
   def include; prefix+'include' end
   def share; prefix+'share' end
 
-  # generally we don't want these to be inside the keg
+  # generally we don't want var stuff inside the keg
   def var; HOMEBREW_PREFIX+'var' end
+  # configuration needs to be preserved past upgrades
   def etc; HOMEBREW_PREFIX+'etc' end
   
   # reimplement if we don't autodetect the download strategy you require