patch 9.1.0852: No warning when X11 registers are not available

Problem:  No warning when X11 registers are not available
          (delvh)
Solution: Output W23 once when connection to X11 clipboard/selection
          is not possible, mention in the documentation, that register 0
          will be used instead

Vim silently uses the 0 register, when clipboard or selection register * or +
are not available. This might be a bit unexpected for the user.

So let's just warn once when this happens.

fixes: #14768
closes: #16013

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/gui.txt b/runtime/doc/gui.txt
index 071de9c..4fdfc0f 100644
--- a/runtime/doc/gui.txt
+++ b/runtime/doc/gui.txt
@@ -1,4 +1,4 @@
-*gui.txt*       For Vim version 9.1.  Last change: 2024 Jul 17
+*gui.txt*       For Vim version 9.1.  Last change: 2024 Nov 07
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -474,6 +474,8 @@
 register is the same as the "* register.  Thus you can yank to and paste the
 selection without prepending "* to commands.
 
+See also |W23|.
+
 ==============================================================================
 5. Menus						*menus*