`Hash#extract!`: Clarify its docs and assert remaining value in `test_extract_nils`

This change emphasizes a remaining value after applying `extract!` on
a hash.

activesupport/lib/active_support/core_ext/hash/slice.rb

@@ -18,8 +18,9 @@ class Hash
 
   # Removes and returns the key/value pairs matching the given keys.
   #
-  #   { a: 1, b: 2, c: 3, d: 4 }.extract!(:a, :b) # => {:a=>1, :b=>2}
-  #   { a: 1, b: 2 }.extract!(:a, :x)             # => {:a=>1}
+  #   hash = { a: 1, b: 2, c: 3, d: 4 }
+  #   hash.extract!(:a, :b) # => {:a=>1, :b=>2}
+  #   hash                  # => {:c=>3, :d=>4}
   def extract!(*keys)
     keys.each_with_object(self.class.new) { |key, result| result[key] = delete(key) if has_key?(key) }
   end

activesupport/test/core_ext/hash_ext_test.rb

@@ -388,11 +388,13 @@ class HashExtTest < ActiveSupport::TestCase
   def test_extract_nils
     original = { a: nil, b: nil }
     expected = { a: nil }
+    remaining = { b: nil }
     extracted = original.extract!(:a, :x)
 
     assert_equal expected, extracted
     assert_nil extracted[:a]
     assert_nil extracted[:x]
+    assert_equal remaining, original
   end
 
   def test_except