patch 8.2.4981: it is not possible to manipulate autocommands

Problem:    It is not possible to manipulate autocommands.
Solution:   Add functions to add, get and set autocommands. (Yegappan
            Lakshmanan, closes #10291)
diff --git a/runtime/doc/usr_41.txt b/runtime/doc/usr_41.txt
index 54af80c..162eb9d 100644
--- a/runtime/doc/usr_41.txt
+++ b/runtime/doc/usr_41.txt
@@ -925,6 +925,11 @@
 	reltimestr()		convert reltime() result to a string
 	reltimefloat()		convert reltime() result to a Float
 
+Autocmds:					*autocmd-functions*
+	autocmd_add()		add a list of autocmds and groups
+	autocmd_delete()	delete a list of autocmds and groups
+	autocmd_get()		return a list of autocmds
+
 			*buffer-functions* *window-functions* *arg-functions*
 Buffers, windows and the argument list:
 	argc()			number of entries in the argument list