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/sautest/autoload/auto9.vim b/src/testdir/sautest/autoload/auto9.vim
new file mode 100644
index 0000000..c89ce3a
--- /dev/null
+++ b/src/testdir/sautest/autoload/auto9.vim
@@ -0,0 +1,9 @@
+vim9script
+
+func auto9#getsome()
+  return 'some'
+endfunc
+
+def auto9#add42(count: number): number
+  return count + 42
+enddef