patch 8.0.1706: cannot sent CTRL-\ to a terminal window

Problem:    Cannot sent CTRL-\ to a terminal window.
Solution:   Make CTRL-W CTRL-\ send CTRL-\ to a terminal window.
diff --git a/runtime/doc/terminal.txt b/runtime/doc/terminal.txt
index 551a374..63217c2 100644
--- a/runtime/doc/terminal.txt
+++ b/runtime/doc/terminal.txt
@@ -72,6 +72,7 @@
 
 Special in the terminal window:			*CTRL-W_.*  *CTRL-W_N* 
 	CTRL-W .	send a CTRL-W to the job in the terminal
+	CTRL-W CTRL-\	send a CTRL-\ to the job in the terminal
 	CTRL-W N	go to Terminal-Normal mode, see |Terminal-mode|
 	CTRL-\ CTRL-N   go to Terminal-Normal mode, see |Terminal-mode|
 	CTRL-W " {reg}  paste register {reg}		*CTRL-W_quote*