patch 8.1.1068: cannot get all the information about current completion

Problem:    Cannot get all the information about current completion.
Solution:   Add complete_info(). (Shougo, Hirohito Higashi, closes #4106)
diff --git a/runtime/doc/usr_41.txt b/runtime/doc/usr_41.txt
index 07d11f8..31beded6 100644
--- a/runtime/doc/usr_41.txt
+++ b/runtime/doc/usr_41.txt
@@ -834,6 +834,7 @@
 	complete()		set found matches
 	complete_add()		add to found matches
 	complete_check()	check if completion should be aborted
+	complete_info()		get current completion information
 	pumvisible()		check if the popup menu is displayed
 
 Folding:					*folding-functions*