patch 9.1.1408: not easily possible to complete from register content

Problem:  not easily possible to complete from register content
Solution: add register-completion submode using i_CTRL-X_CTRL-R
          (glepnir)

closes: #17354

Signed-off-by: glepnir <glephunter@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/vi_diff.txt b/runtime/doc/vi_diff.txt
index 46db57a..ae14968 100644
--- a/runtime/doc/vi_diff.txt
+++ b/runtime/doc/vi_diff.txt
@@ -338,6 +338,7 @@
 	|i_CTRL-X_CTRL-D|	definitions or macros
 	|i_CTRL-X_CTRL-O|	Omni completion: clever completion
 				specifically for a file type
+	|i_CTRL-X_CTRL-R|	words from registers
 	etc.
 
 Long line support.					|'wrap'| |'linebreak'|