patch 8.2.3389: cannot stop insert mode completion without side effects

Problem:    Cannot stop insert mode completion without side effects.
Solution:   Add CTRL-X CTRL-Z. (closes #8821)
diff --git a/runtime/doc/index.txt b/runtime/doc/index.txt
index 683afe4..8c114a2 100644
--- a/runtime/doc/index.txt
+++ b/runtime/doc/index.txt
@@ -168,6 +168,7 @@
 |i_CTRL-X_CTRL-Y|	CTRL-X CTRL-Y	scroll down
 |i_CTRL-X_CTRL-U|	CTRL-X CTRL-U	complete with 'completefunc'
 |i_CTRL-X_CTRL-V|	CTRL-X CTRL-V	complete like in : command line
+|i_CTRL-X_CTRL-Z|	CTRL-X CTRL-Z	stop completion, keeping the text as-is
 |i_CTRL-X_CTRL-]|	CTRL-X CTRL-]	complete tags
 |i_CTRL-X_s|		CTRL-X s	spelling suggestions