Update Hash#compact! documentation [ci skip]

* hash.c (rb_hash_compact_bang): [DOC] update the case if no
  changes were made.  [ruby-core:82591] [Bug #13855] [Fix GH-1692]

Author:    Lucas Buchala <lucasbuchala@gmail.com>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59719 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

hash.c

@@ -2819,10 +2819,10 @@ rb_hash_compact(VALUE hash)
 
 /*
  *  call-seq:
- *     hsh.compact! -> hsh
+ *     hsh.compact! -> hsh or nil
  *
  *  Removes all nil values from the hash.
- *  Returns the hash.
+ *  Returns nil if no changes were made, otherwise returns the hash.
  *
  *     h = { a: 1, b: false, c: nil }
  *     h.compact!     #=> { a: 1, b: false }