Docs: Improve example in merge function

This commit replaces the example in the merge function with a much clearer
one. It also mentions that the rightmost value wins in the event of duplicated
hash keys.

lib/puppet/parser/functions/merge.rb

@@ -5,9 +5,12 @@ module Puppet::Parser::Functions
     For example:
 
         $hash1 = {'one' => 1, 'two', => 2}
-      $hash1 = {'two' => 2, 'three', => 2}
+        $hash2 = {'two' => 'dos', 'three', => 'tres'}
         $merged_hash = merge($hash1, $hash2)
-      # merged_hash =  {'one' => 1, 'two' => 2, 'three' => 2}
+        # The resulting hash is equivalent to:
+        # $merged_hash =  {'one' => 1, 'two' => 'dos', 'three' => 'tres'}
+
+    When there is a duplicate key, the key in the rightmost hash will "win."
 
     ENDHEREDOC