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/insert.txt b/runtime/doc/insert.txt
index f3d92b2..553183d 100644
--- a/runtime/doc/insert.txt
+++ b/runtime/doc/insert.txt
@@ -1,4 +1,4 @@
-*insert.txt*    For Vim version 9.1.  Last change: 2025 May 08
+*insert.txt*    For Vim version 9.1.  Last change: 2025 May 26
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -649,6 +649,7 @@
 11. omni completion					|i_CTRL-X_CTRL-O|
 12. Spelling suggestions				|i_CTRL-X_s|
 13. keywords in 'complete'				|i_CTRL-N| |i_CTRL-P|
+14. words from registers				|i_CTRL-X_CTRL-R|
 
 Additionally, |i_CTRL-X_CTRL-Z| stops completion without changing the text.
 
@@ -1019,6 +1020,21 @@
 			completion, for example: >
 				:imap <Tab> <C-X><C-V>
 
+
+Completing words from registers				*compl-register-words*
+							*i_CTRL-X_CTRL-R*
+CTRL-X CTRL-R		Guess what kind of item is in front of the cursor from
+			all registers and find the first match for it.
+			Further use of CTRL-R (without CTRL-X) will insert the
+			register content, see |i_CTRL-R|.
+			'ignorecase' applies to the matching.
+
+	CTRL-N		Search forwards for next match.  This match replaces
+			the previous one.
+
+	CTRL-P		Search backwards for previous match.  This match
+			replaces the previous one.
+
 User defined completion					*compl-function*
 
 Completion is done by a function that can be defined by the user with the