patch 9.1.0868: the warning about missing clipboard can be improved

Problem:  the warning about missing clipboard can be improved
          (after v9.1.0852)
Solution: use different warnings depending on whether or not clipboard
          support is included in Vim, update related documentation

Improve the Warnings about missing clipboard registers

- Make it translatable
- Use a different warning, when clipboard support was not compiled in
- add a reference to :h W24
- explain in more detail the error message

closes: #16069

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/message.c b/src/message.c
index 5853e47..95bd726 100644
--- a/src/message.c
+++ b/src/message.c
@@ -56,7 +56,6 @@
 static int  verbose_did_open = FALSE;
 
 static int  did_warn_clipboard = FALSE;
-static char *warn_clipboard = "W23: Clipboard register not available, using register 0";
 
 /*
  * When writing messages to the screen, there are many different situations.
@@ -4071,7 +4070,11 @@
 {
     if (!global_busy && !did_warn_clipboard)
     {
-       msg(_(warn_clipboard));
+#ifdef FEAT_CLIPBOARD
+       msg(_("W23: Clipboard register not available, using register 0"));
+#else
+       msg(_("W24: Clipboard register not available. See :h W24"));
+#endif
        did_warn_clipboard = TRUE;
     }
 }
diff --git a/src/testdir/test_registers.vim b/src/testdir/test_registers.vim
index f2d38d8..732cc60 100644
--- a/src/testdir/test_registers.vim
+++ b/src/testdir/test_registers.vim
@@ -1052,7 +1052,7 @@
     new
     call append(0, "text for clipboard test")
     let mess = execute(':norm "*yiw')
-    call assert_match('W23', mess)
+    call assert_match('W24', mess)
     bw!
   endif
 endfunc
diff --git a/src/version.c b/src/version.c
index d4a4e97..b4eb429 100644
--- a/src/version.c
+++ b/src/version.c
@@ -705,6 +705,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    868,
+/**/
     867,
 /**/
     866,