Update doc of Hash#slice [ci skip]

- Add arguments to call-seq
- Add sample of multiple keys
- Add sample: hash does not contain key

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60353 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

hash.c

@@ -1326,13 +1326,14 @@ rb_hash_reject(VALUE hash)
 
 /*
  *  call-seq:
- *     hsh.slice -> a_hash
+ *     hsh.slice(*keys) -> a_hash
  *
  *  Slices a hash to include only the given keys.
  *  Returns a hash containing the given keys.
  *
  *     h = { "a" => 100, "b" => 200, "c" => 300 }
- *     h.slice("a")  #=> {"a" => 100}
+ *     h.slice("a")      #=> {"a"=>100}
+ *     h.slice("c", "d") #=> {"c"=>300}
  */
 
 static VALUE