update doc

Library/Homebrew/formula.rb

@@ -247,9 +247,10 @@ class Formula
     end
   end
 
-  # sometimes the cleaner breaks things
+  # sometimes the formula cleaner breaks things
   # skip cleaning paths in a formula with a class method like this:
-  #   skip_clean bin/"foo", lib/"bar"
+  #   skip_clean "bin/foo", "lib"bar"
+  # keep .la files with:
   #   skip_clean :la
   def skip_clean? path
     return true if path.extname == '.la' and self.class.skip_clean_paths.include? :la