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/usr_24.txt b/runtime/doc/usr_24.txt
index 72f43f0..250bd17 100644
--- a/runtime/doc/usr_24.txt
+++ b/runtime/doc/usr_24.txt
@@ -187,6 +187,7 @@
 	CTRL-X CTRL-D		macro definitions (also in included files)
 	CTRL-X CTRL-I		current and included files
 	CTRL-X CTRL-K		words from a dictionary
+	CTRL-X CTRL-R		words from registers
 	CTRL-X CTRL-T		words from a thesaurus
 	CTRL-X CTRL-]		tags
 	CTRL-X CTRL-V		Vim command line