patch 8.2.0991: cannot get window type for autocmd and preview window

Problem:    Cannot get window type for autocmd and preview window.
Solution:   Add types to win_gettype(). (Yegappan Lakshmanan, closes #6277)
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index 4ab8bbd..fe2e94e 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -10864,7 +10864,10 @@
 
 win_gettype([{nr}])					*win_gettype()*
 		Return the type of the window:
+			"aucmdwin"	autocommand window. Temporary window
+					used to execute autocommands.
 			"popup"		popup window |popup|
+			"preview"	preview window |preview-window|
 			"command"	command-line window |cmdwin|
 			(empty)		normal window
 			"unknown"	window {nr} not found