tty: Print very descriptive message about shell job dump

First thing people typically do with criu is run something
in shell and try to dump it. This is typicall pitfall, as
in that case we face external sid and tty problem. Print a
message on stdout about this and advice wiki page to read.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

tty.c

@@ -1022,11 +1022,18 @@ int dump_verify_tty_sids(void)
 
 			if (!item || item->pid.virt != dinfo->sid) {
 				if (!opts.shell_job) {
-					pr_err("Found dangling tty with sid %d pgid %d (%s) on peer fd %d. "
-					       "Consider using --" OPT_SHELL_JOB " option.\n",
+					pr_err("Found dangling tty with sid %d pgid %d (%s) on peer fd %d.\n",
 					       dinfo->sid, dinfo->pgrp,
 					       tty_type(dinfo->major),
 					       dinfo->fd);
+					/*
+					 * First thing people do with criu is dump smth
+					 * run from shell. This is typical pitfall, warn
+					 * user about it explicitly.
+					 */
+					pr_msg("Task attached to shell terminal. "
+						"Consider using --" OPT_SHELL_JOB " option. "
+						"More details on http://criu.org/Simple_loop\n");
 					ret = -1;
 				}
 			}