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_x11.txt b/runtime/doc/gui_x11.txt
index dbd4b10..0658bfa 100644
--- a/runtime/doc/gui_x11.txt
+++ b/runtime/doc/gui_x11.txt
@@ -1,4 +1,4 @@
-*gui_x11.txt*   For Vim version 9.1.  Last change: 2024 Apr 22
+*gui_x11.txt*   For Vim version 9.1.  Last change: 2024 Nov 11
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -708,6 +708,14 @@
 Note that the text in the "+ register remains available when making a Visual
 selection, which makes other text available in the "* register.  That allows
 overwriting selected text.
+
+								*W23*
+When you are yanking into the "* or "+ register and Vim cannot establish a
+connection to the X11 selection (or clipboard), it will use register 0 and
+output a warning:
+
+  Warning: Clipboard register not available, using register 0 ~
+
 							*x11-cut-buffer*
 There are, by default, 8 cut-buffers: CUT_BUFFER0 to CUT_BUFFER7.  Vim only
 uses CUT_BUFFER0, which is the one that xterm uses by default.