patch 8.2.1325: Vim9: using Vim9 script for autaload not tested

Problem:    Vim9: using Vim9 script for autaload not tested.
Solution:   Add a test.  Update help.
diff --git a/src/testdir/test_autoload.vim b/src/testdir/test_autoload.vim
index b8c4fa2..57967bf 100644
--- a/src/testdir/test_autoload.vim
+++ b/src/testdir/test_autoload.vim
@@ -17,3 +17,8 @@
   source sautest/autoload/sourced.vim
   call assert_equal(1, g:loaded_sourced_vim)
 endfunc
+
+func Test_autoload_vim9script()
+  call assert_equal('some', auto9#getsome())
+  call assert_equal(49, auto9#add42(7))
+endfunc