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>

src/message.c

@@ -56,7 +56,6 @@ static FILE *verbose_fd = NULL;
 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 @@ msg_warn_missing_clipboard(void)
 {
     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;
     }
 }