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
 
diff --git a/runtime/doc/insert.txt b/runtime/doc/insert.txt
index 1caf084..18b40f7 100644
--- a/runtime/doc/insert.txt
+++ b/runtime/doc/insert.txt
@@ -640,6 +640,8 @@
 12. Spelling suggestions				|i_CTRL-X_s|
 13. keywords in 'complete'				|i_CTRL-N| |i_CTRL-P|
 
+Additionally, |i_CTRL-X_CTRL-Z| stops completion without changing the text.
+
 All these, except CTRL-N and CTRL-P, are done in CTRL-X mode.  This is a
 sub-mode of Insert and Replace modes.  You enter CTRL-X mode by typing CTRL-X
 and one of the CTRL-X commands.  You exit CTRL-X mode by typing a key that is
@@ -1042,6 +1044,12 @@
 			other contexts unless a double CTRL-X is used.
 
 
+Stop completion						*compl-stop*
+
+							*i_CTRL-X_CTRL-Z*
+CTRL-X CTRL-Z		Stop completion without changing the text.
+
+
 FUNCTIONS FOR FINDING COMPLETIONS			*complete-functions*
 
 This applies to 'completefunc' and 'omnifunc'.