array.rb: show examples whether `Array#shuffle!` has side effect or not

Partially revert 54fb8fb62a

array.rb

@@ -4,7 +4,8 @@ class Array
   #
   # Shuffles the elements of +self+ in place.
   #    a = [1, 2, 3] #=> [1, 2, 3]
-  #    a.shuffle! #=> [2, 3, 1]
+  #    a.shuffle!    #=> [2, 3, 1]
+  #    a             #=> [2, 3, 1]
   #
   # The optional +random+ argument will be used as the random number generator:
   #    a.shuffle!(random: Random.new(1))  #=> [1, 3, 2]
@@ -17,7 +18,8 @@ class Array
   #
   # Returns a new array with elements of +self+ shuffled.
   #    a = [1, 2, 3] #=> [1, 2, 3]
-  #    a.shuffle #=> [2, 3, 1]
+  #    a.shuffle     #=> [2, 3, 1]
+  #    a             #=> [1, 2, 3]
   #
   # The optional +random+ argument will be used as the random number generator:
   #    a.shuffle(random: Random.new(1))  #=> [1, 3, 2]